We are hoping to get out our new bike trailer for C this weekend, so I went to Target to try to get her a helmet. She is almost 2 but her head is 95%.

Two questions-
How do I know the size is right? The 1-3 year old one seemed too small, like it wouldn't go all the way down her big head. The next size was 3-5 and seemed awfully big? Now, she will be in a trailer so I feel like the risk of hitting her head is fairly minimal but I still want to be safe. How do I tell if it fits right?

2- with a trailer, vs a seat, do I need a flatter backed helmet or is a regular one going to be fine? Didn't occur to me until I was googling about the size and found some that are flatter in the back, but almost twice the price...

Thought?