What does she need done to her teeth??
If it's cosmetic like braces then NO I wouldn't even consider it. If it was a problem that she was in pain from then I might consider it but your family (hubby & kids) needs to come first and if you can't afford it financially then don't.
All you have to say is that you already have too many financial obligations due to your own family and can't afford it without it negatively impacting on your children. And honestly stuff what they think.