Why do not you accept game instead of play?
Child's play is a set phrase in English meaning simple. I think Duo is correct.
Sure, it's a reasonable choice, but it's pretty annoying how on one sentence in this course they'll insist on the most unnatural literal translation with zero flexibility and on the next they want to have one particular idiomatic translation.
When a translation is wrong, it's wrong. It's not "we want it to be like this". Report it.