tony91 said:
Also make sure it's a Snell certified.
This is a problem, to my knowledge, and based of information that I have read on the Snell Foundation's website, there are only 2 manufacturers that bother having their children's helmets Snell certified. Fly is one, I don't recall the name of the other (it has been 3-4 months since I checked).
Do not buy your child's helmet online. She needs to put it on her head before you take it home. It definitely needs to be snug, although you can put a small amount of padding in it to improve the fit (I have purchased helmets that came with foam inserts).
A dealer that says that there are no helmets for kids under 8 is being a little lazy. Age has little to do with the size of a head. You probably need a child's XS helmet, but a small could fit. If you have other dealers in your area, check them out. I have found that often different shops will have better supplies of a certain type. This is usually based on what they sell the most of.
Basically, find a shop with a good assortment of MX helmets, and take your girl to try a few on. Try different brands, and even different models of each brand.
IMO, training wheels=NO!