A friend has his kids use LTE Apple Watches. He set them up with his iPhone and they don't have them. It supports "Kid" mode on the watches and everything. They can call and text him when ever they want, which is really nice.
How old are the kids? My wife can’t find well fitting watch bands for the smallest Apple Watch due to here thin wrists. Can’t imagine finding some for a kid.
You can find handmade Apple Watch bracelets on Etsy. Alongside a lot of dropshipped crap ones from Aliexpress - but the point is it seems like third parties are able to make bracelets that replicate the mechanical “interface” for the bracelets, so you should be able to find any size you want.