Hello, I think over time I have become more assertive to people when it comes to my kids. I saw that everyone else does whatever pleases them so I should too. I think if you don't want to send your kids (which I think is very reasonable, I wouldn't either!) then you shouldn't.

I'd be polite but firm. I'd say my kids aren't up for it and we will visit you .... Don't give excuses or justify your decision as I find it doesn't make much difference any way.

One thing to think about, would you let your mother in law tell you how to feed your kids or where to send them to school or how to put them to sleep. This is another decision amongst the millions we make and it isn't up to her. Don't feel bad and stand strong. It would help if your DH agrees then he could just say no also. After time hopefully she will just pester someone else!