We have gone away the last two years in May half term to hotels. The pools and have been unheated and literally freezing cold. I do open water swimming and couldn't get in! It was terrible for the kids as they couldn't use the pool at all.
So this year we are looking at hiring a villa in Mallorca. One of our criteria was a heated pool but they are few and far between on the dates we are looking at. I'm guessing a small private pool heats from the sun a lot more easily than a big hotel pool, but we are concerned if it isn't warm enough the kids won't use the pool. They are brilliant swimmers but not very hardy! They will be 7 and 5.