Foldable child booster seat

1. A foldable child booster seat, comprising:

a seat base that includes a seat portion having a seating surface, and a pair of armrests respectively disposed at left and right sides of said seat base;

a backrest mounted foldably to said seat base;

a support unit movably coupled to said seat base to permit height adjustment of said seat portion relative to said support unit; and

a tray removably mountable to said armrest to dispose said tray in a state of use, and to said support unit to dispose said tray in a stowed state;

said foldable child booster seat being usable as a height-adjustable seat when said tray is disposed in the stowed state.

2. The foldable child booster seat as claimed in claim 1 , wherein one of said armrests includes a rearward inner side formed with a vertically extending elongated guide channel, and further formed with a first positioning space and a second positioning space that are vertically spaced apart with said second positioning space disposed above said first positioning space,

said backrest including a pivot piece that is movable along said elongated guide channel between said first positioning space and said second positioning space, said pivot piece being pivotable in said second positioning space to permit pivoting of said backrest between folded and unfolded positions relative to said seat base,

one of said armrests further including a forward inner side provided with a first engaging part,

said backrest further having an upper side provided with a second engaging part for coupling removably with said first engaging part when said backrest is at said folded position.

3. The foldable child booster seat as claimed in claim 2 , wherein said first positioning space and said second positioning space are respectively disposed at opposite ends of said elongated guide channel and are in spatial communication with said elongated guide channel.

4. The foldable child booster seat as claimed in claim 3 , wherein one of said first engaging part and said second engaging part is formed with a latch cavity, and the other one of said first engaging part and said second engaging part includes a latch for engaging removably said latch cavity.

5. The foldable child booster seat as claimed in claim 4 , further comprising a latch spring for biasing said latch toward said latch cavity.

6. The foldable child booster seat as claimed in claim 3 , wherein said one of said armrests that is formed with said elongated guide channel is further formed with a space-defining wall bordering said second positioning space, said space-defining wall having a shoulder, said pivot piece having an eccentric protrusion that abuts against said shoulder when said backrest is at said folded position.

7. The foldable child booster seat as claimed in claim 6 , wherein said first positioning space and said eccentric protrusion are configured to arrest rotation of said backrest relative to said seat base when said backrest is at said unfolded position.

8. The foldable child booster seat as claimed in claim 3 , wherein said seat base is formed with a handle cavity, said foldable child booster seat further comprising a handle received in said handle cavity and movable between a received position and an extended position relative to said seat base.

9. The foldable child booster seat as claimed in claim 8 , further comprising a releasable coupling mechanism provided on said seat base and said handle for releasably retaining said handle at one of said received position and said extended position.

10. The foldable child booster seat as claimed in claim 9 , wherein said seat base has a handle cavity wall that confines said handle cavity, said releasable coupling mechanism including a coupling part provided on said handle, and a first limit part provided on said handle cavity wall to couple releasably with said coupling part when said handle is at the extended position.

11. The foldable child booster seat as claimed in claim 10 , wherein said handle cavity has an open end through which said handle is extendable, said releasable coupling mechanism further including a second limit part provided on said handle cavity wall, said first limit part and said second limit part being respectively proximate to and distal from said open end of said handle cavity, said second limit part coupling releasably with said coupling part when said handle is at said received position.

12. The foldable child booster seat as claimed in claim 8 , wherein said seat base and said backrest are configured to permit placement of said foldable child booster seat on a support surface with said handle oriented upward when said backrest is at said folded position.

13. The foldable child booster seat as claimed in claim 2 , wherein said pivot piece resiliently projects from a lower lateral side of said backrest and is biased toward said one of said armrests that is formed with said elongated guide channel.

14. The foldable child booster seat as claimed in claim 13 , wherein said backrest is further formed with a horizontally extending slide channel, said pivot piece extending into said slide channel, said backrest further including a manually operable part for moving said pivot piece along said slide channel to disengage said pivot piece from one of said first positioning space and said second positioning space, and a pivot piece spring for biasing said pivot piece toward said one of said armrests that is formed with said elongated guide channel.

15. The foldable child booster seat as claimed in claim 14 , wherein each of said first positioning space and said second positioning space has a depth that is deeper than a depth of said elongated guide channel.

16. The foldable child booster seat as claimed in claim 3 , wherein said backrest includes

a housing that has a rear surface and that is formed with a storage space that opens toward said rear surface, and

a cover pivoted to said housing for covering and uncovering said storage space.

17. The foldable child booster seat as claimed in claim 1 , wherein each of said armrests includes a rearward inner side formed with a vertically extending elongated guide channel, and further formed with a first positioning space and a second positioning space that are vertically spaced apart with said second positioning space disposed above said first positioning space,

said backrest including a pair of pivot pieces each of which is movable along said elongated guide channel of a corresponding one of said armrests between said first positioning space and said second positioning space, each of said pivot pieces being pivotable in said second positioning space of the corresponding one of said armrests to permit pivoting of said backrest between folded and unfolded positions relative to said seat base,

each of said armrests further including a first engaging part,

said backrest further having a pair of second engaging parts for coupling removably with said first engaging parts of said armrests when said backrest is at one of said folded and unfolded positions.

18. The foldable child booster seat as claimed in claim 17 , wherein said backrest is further formed with a horizontally extending slide channel, said pivot pieces extending into said slide channel, said backrest further including a pair of manually operable parts, each of which is operable to move a corresponding one of said pivot pieces along said slide channel to disengage the corresponding one of said pivot pieces from one of said first positioning space and said second positioning space of a corresponding one of said armrests, and a pair of pivot piece springs each biasing a corresponding one of said pivot piece toward the corresponding one of said armrests.

19. The foldable child booster seat as claimed in claim 1 , wherein said seat base is formed with a locking hole, said backrest being formed with a locking protrusion that is engageable removably with said locking hole when said backrest is at said unfolded position.