Biding their time to reveal their hidden depths

Feigning weakness is an ancient strategy for deceiving your enemies.

This can also be called holding out on someone.

After she sunk a three cushion bank shot like a pro, he smiled and said, "You've been holding out on me."

There's also:

Holding one's cards close to their vest is similar to your request. It means keeping ones tactics, plans or intentions secret until you're willing to reveal them.

And, there's the notion of hustling which is a term that originated in billiards. A hustler is a good player who usually hides their skill to get lesser players to play them for higher stakes.

But, to be honest, I'd typically just go for the phrase you used to ask your question.

Your daughter is just biding her time or waiting for her moment.