Red Trace Grom Helmet - Kids' - 09/10 - 2009 BCS

I work at a ski shop and we carry this helmet. The small and medium don't seem to fit a lot of kids, but the large fits many and it is a very popular helmet.
The large seems to fit kids of all ages and when the large is too big the medium is often too small. I recommend an adjustable helmet for when the large is too big. Although your son is 7, it is not uncommon for young children to have big heads. I would think that an adult medium would be too big for a 7 year old. The helmet should fit well and snug with the chin strap open, but of course closed when using it.
Could your mother have returned with The Giro G9 Junior? The Giro G9 junior in a small or medium might fit, but unless he has a huge head, the adult medium would be too big, especially if you have to adjust the chin strap to make it stay on. If you think that the helmet is too big, then it probably is. Measuring his head isn't very accurate because head shapes are different and helmets in the same brand (and other brands) often have different head shapes. ex: the Red Avid fits entirely different than the Red Theory as well as the Giro Fuse as compared to the Giro G10.
When a helmets fits properly it will feel snug all around. The helmet will sit between above his eyebrows and the middle of his forehead. When you move the helmet back and forward it will grab the skin on his head you will see it moving his eyebrows up and down. This should be done with the helmet unbuckled.

Giro and Red do their kids sizing differently. The Giro Junior G9 is available in small and medium and it is adjustable. The small is about the size of the Red Trace Grom L but adjusts to be slightly smaller and slightly bigger, and the medium is even bigger than that but they are shaped differently. The small fits me and the Adult G9 small also fits me.

I have a small head and I am a 22 year old female and I usually wear an adult small or adult x-small. I have this helmet in a large and it fits me great. I actually got it for my mom who has a slightly smaller head than I do.

I will measure his head -- but the size charts don't seem to indicate kid vs adult sizing and I was under the impression that it's different. Is that not the case? Are there kids' size charts I've missed somewhere?

The best thing to do would be to measure the circumference of his head and see what size he fits into per the sizing chart. Then you should get the correct size.

It is ASTM approved as this is the protection standard test.

yes, kids and youth large are the same size. A large men's is a different size. The best bet it to follow the sizing chart above and measure your kids head with a flexible tape.

Its hard to say if large will fit both kids as all kids have different size melons :)