How do you write, "How infantile!" Or "child's play" in French?

Started this discussion. Last reply by Kathie Hagen Nov 30, 2009. 1 Reply 0 Likes

I heard on an Agatha Christie book Hercule Poirot say, "Quel enfantin____!" Sounded like quel enfantinillage or quel enfantinage but I can't find that anywhere. Thanks!