1) If girl who is less than twelve years and six months old makes a vow which involves physical distress, her father may annul her vow by saying three times to her “Mufar Lach!”. This is only true on condition that he annuls it on the day that he finds out about it. This means that if he finds about it at night, he may annul it during that night and the following day. If he found out in the middle of the day, he may only annul it until the end of that day. (Kitzur Shulchan Aruch 67).
2) If at first the father agreed to the vow, even if the agreement was only in his mind and he had said nothing, he can no longer annul it for her. (ibid).