Speaking from my own perspective, I would not administer Miracle-Mineral-Supplement to an infant unless it was a life or death scenario. And I don't mean to sounding as though I'm debating you on the issue, this is simply my opinion on matters. And here's why;
1. We have no real knowledge on the consequences of Miracle-Mineral-Supplement at the levels used to fight disease on the human body.
2. As adults, we assume the inherent responsibilities for our actions.
The gist of it is, that we(as adults) take Miracle-Mineral-Supplement as a last ditch effort to fight off disease or illness. And so, I'd say that under the circumstances, I wouldn't consider it wise to run trials on a 5 yr old with MMS.
However, have you considered a mouthwash and getting her to rinse her mouth a few times times per day(after meals)? Thats might prove to be a safe compromise.
Unfortunately, I have no knowledge of topical or mouth rinse mixtures myself, and so perhaps someone else here could chime-in and lend a hand?