I think it is a good idea, especially with firearm expertise credentials such as former MP, LEO, or graduate of training school such as Gunsite.
The only restriction I would apply is female only. Statistically most sex abuse is done by us men. We have seen two cases involving friends who trusted male friends who eventually molested the kids. Just not worth risking the odds for this specific situation. We had a firm family policy of only female babysitters, and then rarely used one at all.
But a female, background checked, trained and armed babysitter would be a great premium service. Or an armed female family friend whom we knew their skill and safety level.