Sounds like she is just trying to cause problems !! I think I would prob just go on caring for the child until you were supposed to . If she wants to leave early then she is making a choice to lose her money . There is nothing wrong with you going to the freezer . I think it would be common sense that there are times when children are unsupervised ! Does she think you take them all in to the bathroom ??
I say diffuse , I wouldn't take it serious , I would def give her the choice to go and make her understand you are willing , able and open to look after her child , therefore she would be losing money not you !!!
I think if you return money it looks like she has cause for concern .
I hope the dad will be more realistic !!!
Good luck !!!