I've not been walking around Kannenfeld park since they redid it, but they used to have a paddling pool there.
I know it's quite a trip in the opposite direction, but there is a park in Riehen right near their Gemeinde house (between the Riehen Dorf tram stop on the 6 line and the train station) which does have a paddling pool. M-i-l and I can be found there sometimes with the nephews on particularly hot days.