Tell my straight - it's not cute, she's not being nice. She's deliberating antagonizing the younger children and it won't be tolerated. If Mom cannot see it for what it is and get on board with really addressing it (not pretending to agree to your face and doing nothing behind your back), then she needs to find somewhere else. You have a responsibility to all the children and you aren't willing to risk several clients to save her daughter.

When Mom makes these comments, what do you say?

I'd be replying on the spot - how can you consider that a cuddle, it's an out right headlock. How can you consider her soothing when she took the soother and deliberating made the child cry in order to return it?

Bottom line - you are going to have a hard job getting this child in line with Mom sabotaging you all the way.