There's a difference between acting childish and being child-like.
When we're being child-like, we are in-tune with our joy, some might say naive, but nonetheless, we're happy to be there and in the moment. Feeling the wonder of something new and amazing, seeing a work of art, the beauty of nature, "there's Grandma!" or enjoying the sun on your face as you ride your bike. Perhaps we don't squeal with glee like when we were young, but the feeling is clearly linked to those childhood moments. The anticipation of something exciting is another childhood feeling - that can't quite go to sleep I'm so pumped up rush is a child-like feeling that can kicks lot of endorphins into our grown up bodies. It's possible to feel that way again if we allow ourselves the wonder and joy that was so freely expressed as a child.
Somewhere along the way, we replace many of those anticipatory moments with worry and angst. Growing up doesn't mean replacing joy with anxiety, but it does require less (and less) childish behavior.
Acting childish: first notice that you'll have to act because you are not a child anymore.
Try this one:
I want my own way! Screw up your face 'til you look fierce and see if that tantrum look still gets the job done (at home or at work) . . . probably not.
I want this now! Give in to yourself and buy what you don't need or can't afford. The instant gratification will soon give way to the weight of the bill or the shame you feel when that shiny new item sits in the closet.
I don't want green food, I want ice cream. You're in charge now - do it everyday if you want. See if this creates child-like wonder or the need for a new wardrobe.
When choosing between acting childish or being child-like, choose be.