You know what amazes me most about very young children? They are totally incapable of holding a grudge.
There is this one time when Gab accidentally knocked out her Papa's Coke at a restaurant and most of it spilled onto my jeans and shoes. I was so pissed, I dragged her by the arm, marched her into the restroom and alternately hissed and shouted at her when she chose to be cheery & playful while I washed her sticky hands in the sink. Back at our table, I lost it again (yes, I was in pretty foul mood that day) when she wouldn't swallow her spaghetti because, heaven forbid, there was a teeny tiny bit of meat with it. As she was spitting out the mouthful into the tissue I was holding out to her, strands of spaghetti fell on her jeans leaving streaks of red. Back to the restroom we marched where I dabbed at the stain with wet tissue, purposely dabbing a bit too hard. Even after my despicable treatment of her for such small accidents, she still burst out of the restroom singing happily to herself. When her father admonished her to apologize (again) to me and to give me a hug, she obediently did so completely unaware of how unjustly I've just treated her.
Have you ever had one of those moments when you know you are being a total jerk but you can't help yourself anyway? With Gab, I get a lot of those moments. A lot. And bless her forgiving, amnesiac, kind, little big heart because she never keeps tab. Like an adult would.
So I think the line 'acting like a child' shouldn't always be used in a negative context. Because little children have many fine qualities we could all emulate. Maybe it would do us good to act like children in certain situations, like Gab in the story above. So quick to forget, there's nothing to forgive.
Children ought to be saying to one another: 'Stop acting like an adult' which would be tantamount to saying: Stop being an ***hole. Now, that would be more accurate.