What do you call someone who gets along with children?
Maternal and Paternal work in some cases.
Maternal
-- (of feelings) typical of a caring mother; motherly.Paternal
-- of or appropriate to a father.
I suggest avuncular. Here is part of a discussion of the term at Merriam-Webster Online:
Avuncular derives from the Latin noun avunculus, which translates as "maternal uncle," but since at least the 1830s English speakers have used "avuncular" to refer to uncles from either side of the family or even to individuals who are simply uncle-like in character or behavior.