Your mother may be afraid of you getting angry to her, scold her and she takes revenge on the kids.
Perhaps, you talk to her calmly in presence of your mother and husband (making it th consensus of the family) stating that she cannot go out without your permission. If she needs to go out, she has to call you. Don't ask your mother and your mother would say NO.
But if your mother tolerates and spoils her, it would be difficult. Your mother has to understand it is not you not letting her go out, but she does not have to during working hours. You let her go on holiday.
It seems you have to convince your mother and make her stand on your side first.
If your mother does not stand on your side, the helper can just ignore you as your mother may cover her.
You cannot monitor her all the time when you are at work.
I think your mother loves the kids very much. You may pinpoint to your mother that if the helper goes out more often, she will pay less attention on taking care of kids; she may hurt the kid as a result of negligence when she is out chating with friends... Hope you can bring your mother to your side.
You ask your helper what is her need to go out - hope she won't say you she wants to see friends and chat. Making her to know there is NO need as far as her duties are concerned.