How can we legally make sure he sends the kid back to her mom after vacation? Can we do this with some kind of an agreement before sending the kid to her father for a visit?
If yes, What kind of action can we take if he fails on the agreement?
It is an appreciable idea that you enter into an agreement for this arrangement as expressed by you.
But the hidden danger is tht he is not person to abide by any such obligation.
What is he refuses to send his daughter after the completion of vacation, you cannot force him to leave her nor you can take the help of police on such issues, because he is the father of the child having equal rights over the child.
No such agreements are enforceable in law.
Therefore to avoid such risks you may better not to allow the child to be ther with him even during vacation, if at all he is so much interested let him file a child custody case before court and obtain visitation rights as per his choice and desire.