Faye, I have absolutely no worries regarding how my children will treat me when I'm older. We're a family who keep the older members with us, and my daughter and her husband are currently building a new family home that has all the facilities any older person will need. I think her MIL will be moving in with them as soon as its ready. Then there's one of my sons who also has a house that was deliberately bought big enough so people who need looked after can be. Its just the way we do it, we look after the elderly and those who cant look after themselves.
You wish you had spent more time with your mum and been a bit more patient, and it appears to be what is shaping your opinion on the OP's situation. Not everyone has the regrets you do, perhaps because they did spend enough time with their loved ones and managed to be patient. Its probably why they can encourage the OP to help her daughter and children have what sounds like a much needed and deserved holiday.
I would love to have more time with my mum but not because I didn't give her enough of my time when she was alive. We were always together, and I would just like time with her again to catch up on who's doing what, introduce her to my youngest who she never knew, introduce her to her great grandchildren, show her how far we've come since we last saw her, and if we could swing a night in front of the TV with a bar of chocolate and cuppa it would be just grand.
I've no regrets and no ulterior motives.
As for Christmas Day at Disney, in the actual park, or one of the water parks, it would be really crowded but great fun. There comes a time in life when granny has to be mature enough and wise enough to say, off you go and have a great time, I will be fine, this is for the greater good. This is one of those times and all the more-so because this particular granny is able to have a social life and can put other plans in place. I doubt the idea of the holiday would be under consideration if this wasn't the case.