...If she is actually teaching the lesson...

I don't have teenagers, so feel free to ignore me! lol But honestly, sounds to me like you are upset and angry and personally I don't like the way you said to your friend when the incident happen that it was "her decision to use DSD again or not" and now you are asking her not to outright.
Your trust issues are not your friends issues to be had or carried. You need to address them with DSD. If you are simply preventing her from working for anyone else because you have issues, I think it send the wrong message to DSD - especially if you haven't informed her you have asked your friend not to ask her to babysit until you feel she can be trusted again.


Appears you need to talk to your friend and work out those issues and work on keeping those issues form your DSD's trust issues iykwim.
Must be tough for you at the moment, I hope DSD will come around and help out and you can all relax and settle in a little bit further.
I agree with this. It doesn't sound fair. You are chopping and changing the rules, without warning, all because you are emotional and then you are trying to justify it after the fact to make yourself feel better about doing so.

You are hurt and cranky (understandably) at DSD. If you wanted to make an issue of this, you should have done so at the time. It is not fair to treat her one way after the incident and then change the rules again later on.

You are hurt and cranky at your friend. You are trying to punish your friend, by taking something away from her. And you don't want to feel bad so you are trying to justify it as rational reasonable behaviour. It's not. It is tit-for-tat.

Both of them would have good reason to be p'ed off with you now. But then your aim does seem to be to make them as cranky and hurt as you currently are.

Sorry if that sounds harsh, but you did say you wanted opinions.