I would not recommend going to Yellowstone and staying in timeshares. We stayed in a timeshare for a week in West Yellowstone just outside the park gate. There was soooooooooo much driving because the park is so large and so spread out. Our daughter was elementary school age and it was just too much time in the backseat of the car for her. If you ever do go to Yellowstone, Yosemite, or other National Parks, it would be better to just pay the $$ and stay INSIDE the park, to avoid the huge commutes getting in and out of the park each day.
Also, she was not a fan of our week in the Canadian Rockies (Banff). Same thing -- it was just too much time in the car driving. She will tell you now as an adult that those were the 2 worst timeshare vacations, ever. Kids have a lot of energy, and when you arrive at wherever you have driven to -- they will be happier kiddos if the plan is to do something that they will also find fun and interesting. If they can get out some of their amazing physical energy that would be great, too!
When our daughter was younger her most enjoyable timeshare vacations were: Hawaii -all islands, Caribbean, Mexico -- because there was always the daily swimming and fantastic snorkeling, which she really loved. She also liked big city timeshares with lots to see and do -- New York City, Washington DC, Chicago, San Francisco, Paris, Rome, and Florence. But, it ends up that as an adult, she really is a "big city" person. She won't join us for a week in Vermont, but our next big vacation is a 2-week stay in Barcelona --- and guess who's joining us? Haha. It's all good.
Weather wise for May/Memorial Day Week: Southern California weather should be great (but ocean is still too cold for swimming), Hawaii - perfect weather & warm ocean water, plus easier trades because most kids are still in school. Or maybe someplace more exotic like Costa Rica with monkeys, jungles, and ziplines. Lots of resort destinations offer ziplining now, which I'm sure your kids would love!
--- Rene McDaniel