Your child has just heard the story of Krishna and the butter pot. They are processing one thing only. He stole. They want to know whether that's okay.
The first answer is honest, not theological
A 4-year-old asking why Krishna stole butter isn't asking for the divine-play interpretation. They're asking whether stealing is sometimes okay. The honest answer is: it's mischief, his mother is playing along, no one is hungry, no one is hurt. That's enough. That's the answer they came for.
If you reach for leela here, you're answering a question that hasn't been asked yet. Save it. They'll ask.
The second answer is theirs to discover
As children grow, the question changes. Why does a god steal? Is it really stealing if everything already belongs to him? Why do the gopis laugh instead of being angry? Don't preempt this conversation by answering it before they've asked. They'll arrive, around 7 or 8, with a better-shaped version of the question.
The third answer is yours
Different Indian traditions answer this differently. The Bhagavata Purana frames it as leela. Divine play, where what looks like theft is the god showing himself in the most ordinary scenes. Other traditions frame it as a child being a child, and the divinity is incidental to the scene. You can hold both. You can pick one. You can change your mind by the time your child is 12. That's allowed.