I remember learning this, but I can't guarantee it's accurate, that canker sores were cold sores INSIDE your mouth. It's the same virus. If it IS the same virus, you may want to get an as-needed prescription for valtrex. You would take it only when you feel the canker sore for 5 days, I think. It should start to work almost immediately.
I'm not sure if kids are supposed to take it or not, though.
I think, also, what you might want to do is find a medication that stays on soft tissue/membrane. I'm not sure that Abreva will, even if it's a great medication. You really just want to protect the area from food and bacteria. There are some liquid medications (ask the pharmacist) that should be able to adhere to the tissue inside your mouth.
Good Luck.