Well I think a lot of kids start babysitting at 12 or 13. As long as S15 is comfortable and D9 is fine with it, I don't see a problem. S15 should have the landlord's phone number, your number and be fine addressing minor issues but knows what to do in case of an emergency.
Time for your H to give you some credit.
kat
Me-53(and learning!) S24, S21, D18, D17 Just keep swimming, Just keep swimming. Dory