After a player unchecks Power, is there a window after the card resolves for a player to activate a Teemo from hidden?

No. Once Unchecked Power resolves, there is no window to play a Teemo from Hidden until another chain begins. The only opportunity is before Unchecked Power resolves — but if you do that, Teemo is on the battlefield when the 12 damage hits and will die along with everything else.

Here's the sequence: Unchecked Power is played → the opponent can react (including playing a Hidden unit, but it will be hit by the damage) → Unchecked Power resolves, dealing 12 to all units → units die during cleanup → the Chain is empty and the state is Open, but you've already passed the reaction window for that spell and no new game action has created a new chain yet. You cannot squeeze a reaction in after a spell finishes resolving and before the next play opportunity — the game moves to an Open State where the turn player gets priority to act next.