The ornamental design for a foldablechildboosterseat, as shown and described.
1. Foldable child booster seat
1.1: Back
1.2: Perspective
1.3: Front
1.4: Top
1.5: Right
1.6: Left
1.7: Bottom
1.8: Front Folded
1.9: Right Folded
1.10: Perspective
1.11: Back Folded
1.12: Left Folded
1.13: Top Folded
1.14: Bottom Folded
The design is a child booster seat that can be mostly used in car; it can be folded into a smaller size; the armrest can be turned and then hidden in the pad.
The broken lines depict portions of the foldable child booster seat that form no part of the claimed design.