If you want her to go, but she seems reluctant, can you either make her think the tickets are nob-transferable and there for you would lose money (that would have worked for my grandma) or that you need her to watch the two younger kids so that you can take your oldest daughter on some of the rides with height requirements. I bet she feels that she will just be in the way. If you have a good reason for her being there, I bet she would come.
. Unless, of course, she's the kind who'd call and ask herself, lol.