When I was 16 (which was a very long time ago now, in the early 90's) I travelled from Australia to Canada with two connections via the US on Qantas and US Air (SYD - LAX - PHL - YYZ). Different times in a way; however customs was rude and unhelpful at LAX but eventually everything got sorted out and I did not have any issues following signs etc. I'm sure they'll be fine, but I would simplify their experience with a direct flight if at all possible and make sure they're equipped with cell phones just for emergencies or if customs has questions that have roaming available on them (not all the UK prepaid options do).