OT - Child car seats

I have been trying to check out the various regulations on children in
cars for our trip down I-95 and am now very confused. Over here in the
UK both boys (aged 4yo and 6yo) use child seats (made from polystyrene
with a fabric cover - hence light enough to bring as baggage) that use
the adult seatbelts rather than an integral 3 or 5 point harness. Are
these seats legal in the US and particularly in the states that I-95
runs through. I have looked at various sites on the internet and have
come away even more confused....

Can somebody help me, please?

Lizzy

PS Both boys are miniature Houdinis and would be most put out by going
back into what they would see as "baby" seats so would almost certainly
undo the buckles and be out in very short order.
--
Lizzy Taylor
Heywood, Lancashire, UK 53:36:00N 2:06:00E
http://www.thetaylorfamily.org.uk