I would be very careful in your situation. It is my understanding that this foal was a gift to you, correct? You did not pay for her? If so, I would not be telling the mare's owners (your benefactors) what they can and can not do with the foal, unless they are putting baby in danger.
It won't be long until the foal is weaned and you move her to another barn where you will have absolute say. She will still be a baby, not wrecked yet,lol.
I would focus mainly on ensuring the filly is respectful of You when you are with her. I would try to use the utmost respect when dealing with the other owners, especially if your contract is just verbal. Because with just a promise, I would assume that until the filly is weaned and in your hands, she is still theirs.