I did that to my tongue once. It was numb, and I gnawed it up terribly. It healed all by itself very quickly. Mouths, especially children's mouths, do heal very quickly. I would call the dentist back and ask if they have any suggestions, but I'd also bet that it'll heal very nicely all on its own. I would discourage her, obviously, from chewing on it more or messing with it, and let her eat whatever she's comfortable eating until it's feeling better.
If it were me, I'd rinse it with listerine to keep it clean, but I don't know if that's safe for a LO. You'd have to make sure she didn't swallow any. And it would sting like the dickens. I dunno, but I'll bet the dentist's office would.
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