I say take grandma along. I am still trying to convince my MIL to go with us in May. So far no luck though. She just had knee surgery. I told her we will push her in a wheel chair if she wants us too.
We are already going with my dad and his wife, but one of my neighbor's suggested that Disney was a two sets of grandparents sort of vacation.
I think only you can decide. It sounds like it could be good either way. Although, if you are going to be guilt ridden the entire time, it's not worth it.
One more point of view, another one of my neighbors is heading down in a couple weeks. Her son went when he was 2 and is now 4. He told his parents to please call the grandparents and let him stay with them. He doesn't want to go. He remembers being afraid of the characters. I would like to think he will change his mind once he gets there and have a great time.
) She still remembers parts of that trip and our memories will last forever. An added bonus is that she was practically free ~ we were able to stay in a standard room at WL (usually only accomodate 4), no need to purchase park admission, free meals at the buffets, character meals etc. We also had our then 12 year old and then 5 year old. Sometimes we split up to take the oldest on some thrill rides and other times we all went together on rides like Dumbo, Winnie the Pooh etc. I think you should take her ~ it will be worth it!
you'd miss your princess. 
