ANSWER:
It is really very wrong of parents to bring their ill behaved children to the masjid when they know how their children are. The Prophet salla allahu alaihi wa sallam had said that one doesn’t believe until he loves to his brother what he loves for himself. No one loves to be annoyed by children that make noise and disturb the peace in masjid.
No problem in telling them that they’re disturbing others and that they must not bring them.