Is it legal to restrict the number of kids in a store?

I cannot think of any law anywhere that would prohibit this restriction. It gets close to discrimination laws, but does not seem to touch them. The overriding law is usually properly law. The owner of the property has the right to prohibit anyone for any reason or for no reason at all. But that is not absolute.

You could not limit the number of people from a certain country where it is illegal to discriminate in service based upon national origin, that is for sure.