The 'Erlebnisfreibad Sasbachwalden' (5 km) is a public pool beautifully integrated in the hilly landscape. It can be used free of charge by guests of 'Chalet Bergweide'. Your kids will like the 56 meter slide and the fun pool, a 25 meter sports pool with a 1 meter and 3 meter springboard. In addition, you can play beach volleyball, table tennis, and enjoy a paddling pool.
There are also various lakes in the Rhine valley, e.g. Lake Achernsee (15 km) (http://bit.ly/2jjHMfD).
Photos: Gemeinde Sasbachwalden, Stadt Achern