(I realize this thread is a couple weeks old...)
I have our 2 year old wear a helmet in the bike trailer. The 7 year old too, when she came along for the trek cancer ride. To help with the younger one, we have a little ritual for putting on her helmet (she calls it a "hat") that involves patting her helmet (open handed - on the top) and then she does mine, and we do this back and forth a couple of times. Makes it a bit of a game. Also, yes, in the trailer the backrest makes the helmet push forward on her head and is uncomforable. Solution? A small pillow that came with her bedding. It's about 10x10 and leves her enough room beween her and the seat back for the helmet, and is thin enough so it doesn't push her off the front of the seat. Now when we look through cycling magazines she says "is that you daddy?" "no, that's not me- that's someone else" "he's on a bicycle?" "yup, and see? He has his hat on" "yep!"
she has to have her helmet on to ride her scooter, tricycle, or fisher price bicycle outside.