Vent away.

I totally understand how you feel. I am surprised the family has actually stuck around. For myself if my child was not adjusting well to a daycare to the point the provider gave notice I certainly wouldn't want to put him through that and would move on as quickly as I could.

Good luck and yes I agree it would not be a good idea to go back on what you have already done. Let them go and carry on. No guarantees with the next child unfortunately.