It's odd that Numbuh Two's mom approves of him being in the KND, seeing as their existence is to oppose adults.
Addition: She probably thinks they're playing a game!!! If you're gonna ask that, then ask why Numbuh One's dad doesn't mind the giant clubhouse in the tree in their back yard. They don't care. Besides, it's a cartoon.
Other Addition: Besides, the Kids Next Door fight EVIL adults, not their own parents.