The problem with a 3 year old is that before you can turn around they are 6 or 7.. and a lot taller..
I would make it a fairly good size, 45 or 50 by 60 or a 65.. that way it will last longer than the baby quilts we make for newborns. I would want a child to get several years of use out of it as a blanket and then it still be big enough to be a lap quilt. Just my thoughts on what I'd do.. I'm sure they will love whatever size you make it.