It would be nice if it were that simple. It actually takes some convoluted code to find out which resource groups the resource belongs to. Then you'd have some more convoluted code to find out if the user belongs to a user group that has access to any of them.
If there's a complex relationship between User Groups and Resource Groups that changes often, you're better off getting the ACL entries to work. If they're set up properly, this should work:
if ($childresource->checkPolicy('some_permission')) {
/* Ok to view */
}
You just have to make sure the users who shouldn't see the resources don't have the permission. The permission to use will be in the policy attached to the appropriate Resource Group Access ACL entry for the front-end context. The 'view' permission would make sense as long as (anonymous) users don't have it.