Does touching the buttocks of a young female child nullify ablution?

Touching a young girl (who has not reached the age of discretion) does not nullify ablution. Scholars have explicitly stated that touching a young girl does not affect the validity of ablution, especially for someone who is her mahram (unmarriageable kin) and given her age. The touching of a young girl by a man, or a young boy by a woman—meaning those under seven years old—does not nullify ablution. They (the scholars) agreed that the correct view concerning a young child is that ablution is not invalidated. As for touching anything other than the private parts, it does not nullify ablution, even if it is with desire, because one does not typically derive pleasure from it.