La Plage de Saint Jorioz, Annecy

The municipal beach at Saint-Jorioz on Lake Annecy has some good sand in particular. I imagine this place is hugely busy in the summer months. There's a well fenced playground but the main attraction is paddling of course.

The Woods in Argentière

Argentière is 9km from Chamonix. The Petit Balcon Nord is a woodland walk between the two and has a picturesque playground. There's a river beach for paddling in. An unusual thing about this playground is that is impossible to drive to which makes a change.

Lake side Park at Le Vengeron

Between Versoix and Geneva is Le Vergeron. The playground is small but this park is for playing in by the lake side. The park has a  youthful feel with ball games, music and plenty of personal barbecues about.

On the lake in Pregny along from Jardin Botanique

One of the more tranquil and close "beaches" to Geneva is opposite and along from the Jardin Botanique.  I don't know the name of this little park but in has a good buvette and restaurant. The Restaurant Reposoir serves drinks, ice cream, snacks and meals on some very comfy sofas.

The beach  is some sloping concrete but none the less you can paddle.

I've added a new tag Lake Side for playgrounds on the lake.