What's the opposite of "grow up" when used as an interjection?

In casual conversation, one is often told to "Grow up!", ie be more mature and more like an adult. What would be the most accurate phrase to express the opposite sentiment - you need to loosen up, embrace your inner child, appreciate the fun in activities usually seen as frivolous, etc?

• "Grow down" is humorous but I'm not sure if it's correct, and it's definitely not widely used.
• "Loosen up" works but doesn't have quite the same "child-like" connotation.
• "Relax", "chill" and other such common words also fall in the same category, but don't express a need for childishness.

Solution 1:

"Live a little!" would be the answer in my local dialect. That said, this is usually done not as a positive reinforcement towards childishness, but as a negative deterrence from being overly uptight/mature. i.e. "get that prick out of your bum, you're not better than everybody else" or various local variations to the effect of "stop being so uptight"

Solution 2:

"Have some fun!" or "Live your life!" have the meaning of what you want, but don't explicitly imply an age. "Act your age" would only make sense in the right context. I specifically remember "Be a kid for once" from somewhere (maybe a TV show)that would be appropriate for this.