1. No such agreement will be enforceable in law.
If you want to keep them seated at the tip of the chair for ever showing this agreement copy, you can get it notified to them, or else this will not be of any use.
Instead of complicating the things any further, better take her back on the words of the elders from their side.
2. Dont do the mistake of sending such communication, suppose they take it as insult/hurt, then the entire efforts will go waste, instead warn her that she will be sent back to her parents home if she is doing the same mischief again.