When is she nipping? Only when excited? When did she start doing this? Put a leash on her and redirect to something she IS allowed to put her teeth on. If the kids get her all riled up then they need to be taught how to play appropriately with the dog to prevent the biting.
How long has she been with you? Have you attended classes with her?
How much exercise (mental and physical) does she get daily? She may need more physical exercise.
As for the underbite - doesn't matter unless it begins to interfere with her eating or breathing. We had a rescue foster with an underbite and it did not affect her at all except she often appeared to be pouting - her lower lip was pushed out a little, almost like a bulldog
- it was cute!