I have a 2yo and while he loves Disney (we were there in nov, when he had just turned 2), it is a totally different vacation with just a 5yo. He is not a kid who sits well at restaurants, and we like to enjoy nice dinners, so it ended up that either dh or I was always walking around with him during a meal. Don't feel guilty. He will have a wonderful time with grandma. You are so lucky to have a family member who is willing to keep him while you enjoy a special trip with your older son. He will get his turn too one day.