Thanks for all your replies, everyone. Scummy, how are you and your boys? (how nice to be missed, too!). That's a great idea about a 'gated' and fenced-off pool, that hadn't occurred to me. After searching through brochures/internet, I didn't manage to find a single one in Tuscany, unfortunately (we have decided to go there instead of the Lakes), although they seem to be more common in France/Portugal.
Tinker, that's a terrible story, my worst nightmare. And I've only taken ds swimming once! I know his cousins would want to be fairly near a pool or beach, but the villa/pool idea is starting to sound not so great the more I think about it.
Tigermoth, thank you (really) for being doom and gloom, I hadn't played out all the possible scenarios over in my mind, so I needed to read all that! Like you said, there must be someone out there who has stayed with a toddler in a villa with a pool, and had a perfect holiday, but I am now of the conclusion that I don't want to be the guineapig in this experiment.