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?

Humble's directions for mouth and gum care are as follows:
Activate a 10-drop dosage of mms in a clean, dry glass. Add 6-8 ounces of purified or distilled water after waiting for the activation.
POUR the mixture over a soft toothbrush and very GENTLY brush the gums and teeth with it.
Pour some on the toothbrush, saturating it, brushing a small portion of the gums and teeth, pour more solution on the toothbrush, brush another small section, etc., until all of the teeth and gums have been brushed.
Jim did not say the dosage would be less for a child as far as the mouth and gum care, but to be cautious, I would start out with HALF the dosage (5 drops of mms instead of 10) for your child.
The directions for taking it INTERNALLY are different for a child, however, so if you decide to go that route, you will want to check Jim's book for the correct dosages.
All the best!