Children can be expected to act in a childish way.
Note: “Boys will be boys” is often applied to grown men who act childishly.
boys will be boys
One can expect boys to act childishly or misbehave, as in We told the kids not to eat in the living room, but when we got home there was a big mess thereoh well, boys will be boys. This term came from a Latin proverb, translated as "Children [boys] are children [boys] and do childish things." In English it was first recorded in 1589.