I get canker sores a lot too. Lately what seems to bring the quickest pain relief and improvement is mixing acidophilus powder in a little water and using an eye-dropper, putting a few drops right on them -- I try to dry them off first. (I use the eye dropper because I don't care for the taste.) When I have more than a couple or they're unreachable, I add a little more water and swish it around and swallow.
I hope this helps you and your little one!
Michelle