The reason children shouldn't sit in front is because the seatbelt doesn't hold a child properly, so they can slip forward more than an adult would. This puts their head in the perfect position so that an airbag can either a.) break their neck, or b.) suffocate them.

In fact, even in the rear seats, as mbeige has said, they need to be in a properly secured child seat. Even larger children don't fit seatbelts well, and can slip out of adult seatbelts and break their necks when they hit the seat in front of them.