what is the smallest size (in centimeters or child age) that a snowmobile or ATV helmet is made?

[wishiwaswest]

I am trying to find out the smallest size made tha's t is DOT approved for ATV and snowmobile. My ex claims to have an "infant" helmet that he uses when my 1 year old goes on the snowmobile with him.
I live in Michigan and the law requires that all passangers where a DOT approved helmet that fits correctly. Any help would be greatly appreciated.
Thanks!