There isn't a law for booster seats on the island that I am aware of.

With that said, the guidelines are usually 40 pounds is the minimum weight for a belt positioning booster seat - until that point they should still be in a forward facing 5-point harness. They should remain in a belt-positioning booster until they reach 4' 9". The concern being will the lap belt sit across their lap (and not belly) and will the shoulder belt be off of their neck. There are some option out there for very simple and easily portable boosters, like mifold (avoid the bubblebum it didn't work well for us).