Rather late in this discussion, but I would talk to your son / DiL and say that you do not feel able to do this.
Don't talk about your feelings around Kumon, or you'll get into a discussion about it instead of your grand-daughters.
If they say she will get behind, say you are sorry they believe that, but you cannot do this.
If they say you cannot take them on holiday / have them to stay (and they would have to be really daft to say that!) then say you will have to think very carefully about that, need time to do so, and ask them to do the same.
Your grand-daughter sounds old enough to talk to, say that parents get very worried that she should do well, and that the grown-ups are going to talk about it.
And don't worry about your grand-daughter's refusal - at least she has a mind of her own! You can let her know you sympathise without undermining her parents.