I've never been there but as far as beaches go you could try Cottontail Corner beach on the North Saskatchewan river.
Across the river and upstream of Devon. One of the few places you can see a full moon at noon. Probably a few full moons. It is a clothing optional beach so you probably won't want to take older kids there.
For central Alberta The west lake at Twin lakes west of Hwy 2 on Hwy 13 is always clean swimming water. Edmonton diving club has been diving there for years. Typical Alberta campground too. Can't say there is much of a beach though.