I have prayed and acted in advance on a pledge to St. Demiana to heal one of my Sunday School students. She was alright for a month, but then her headaches returned. Why has not God healed her completely for the sake of the child’s faith in Him?
It is great to pray with such faith that God may heal this child but we should not forget that everything we ask should be according to God's will. God answered your prayers and her headache disappeared for a whole month.
Nevertheless, have you considered that God might have another plan for the child that is not revealed for us now? St. Paul pleaded to be healed and God denied him his request "Concerning this thing I pleaded with the Lord three times that it might depart from me and He said to me, "My grace is sufficient for you, for My strength is made perfect in weakness" (2 Cor12:8-9).
Do not tempt the Lord. Even if you act in advance on a promise, this does not mean that God automatically will answer your prayers the way you want it. Continue on praying for her and may God's plan be soon revealed.