My husband had an abscess; it showed up as a pimple-like bump on his gums. We tried different things with varying degrees of success. In the end, though, no matter what we tried, it would never completely go away. He was extremely reluctant to return to the dentist, as the dentist he had been going to had been brushing off his concerns. ("Tooth hurts? Nah, nah, it's fine").
What finally got rid of it for good was a combo of white oak bark powder (capsules) and dandelion tincture. The combo, taken consistently, took care of the problem. The last time he went to his (new!) dentist, they told him there wasn't any sign of the abscess ever being there.
This is especially cool because of how long the abscess was there -- supposedly there's a very real chance that it could have eaten away part of his jaw bone by then, but according to his x-rays, there's no sign of it.
Herbalist Matthew Wood feels that white oak bark helps recalcify bone. Another herbalist mentioned dandelion being good for infections "down to the bone".
I'm not sure about doses with a young child. Something to be aware of: white oak bark is astringent; if she already happens to be suffer from constipation, it can make the condition worse.
I hope you find something that helps -- tooth problems are SO frustrating!