For a boy = Sé JUICIOSO (whee see OH so)
For a girl = Sé JUICIOSA (whee see OH sah)
It literally means "be judicious". But, in Colombia, it means "behave yourself."
Most kids will have heard this a thousand times, and they will be familiar with its meaning and the behavior you expect after they hear it.
Four years ago, a family I know brought home 2 children from Colombia. They asked me over to help teach the kids ESL. The adoptive mother was struggling with the language. When the kids started acting up, I reminded them to be JUICIOSOS. It helped.