The shortest I’d go would be the squier mini which I think it’s 28” and some change. Unless you are going to get a UBass there’s no reason to go any shorter.
Bass guitar is a long instrument at whatever age might as well get used to them. Plus kids will develop their own techniques to deal with their short coming, pun intended. My ex girlfriend was a 5 foot nothing in heels with short tiny fingers who’s a concert pianist it’s amazing how she plays some complex pieces. Of course people at that level makes everything looks easy but man it’s like watching Yoda fights. “Do or do not, there’s no try”.