I would think if you opened a savings account and put the approx cost of phi in every week, you would have her costs pretty much covered. All the phi i know of suck for any dental.
My sister was missing 5 adult teeth, her baby teeth were removed (something to do with them being likely to cause serious issues long term, cant remember the details sorry!!) She had braces to pull her other teeth around to close any gaps and kepther wisdom teeth when they cut thru at 20. The only downside is a couple of her molars that were pulled forward are a different colour to their neighbours but that might just be her teeth.