I think the issue is that the common areas are for the enjoyment of the members, their families, guests and/or tenants. To tell a member that has children that they can't use the common area while at the same time telling members without children that they can use the common area, the Association is not treating everyone the same (i.e. discriminating).
Here is what davis-striling site says about
swimming pools (which I know isn't exactly the same as open land but it is still considered a common area equally owned by all members).
Here, from the same site, is
Dept. of Fair Employment & Housing demand letter about discriminating against children.
From the
Fair Housing in California:
Families with Children. A Manual for Housing Providers, Tenants and Advocates[comment added]
Additionally, an owner or housing manager may not harass or interfere with an individual by withholding services or privileges that are granted to everyone else, when such action is based on the individual’s membership in a “protected
class.” Such privileges might include use of the laundry room, pool, or other facilities [ i.e. common areas], the provision of parking spaces, or prompt repairs of the premises.
Perhaps if the individuals are damaging property, you should let the property owners settle the issue with the parents and/or involve the police rather than pass rules/regs that causes legal issues for the Association.