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.