I might check water quality more so than warmth. On a hot day it's actually kind of nice to swim in colder water anyway, even for kids. And some of these spots can get surprisingly gross, especially in warmer water conditions.
https://www.kelowna.ca/parks-recreat...-water-quality
The over all location and crowd factor would also be a bigger consideration for me. I wouldn't want to hang out at one of the busy beaches in Kelowna but some of the smaller, less crowded ones would be my pick.