The water playground is open between mid-May and mid-September (with temperatures above 22°C).
Wikipedia (German) http://de.wikipedia.org/wiki/Park_Louisa#Waldspielpark_Louisa
More information on Frankfurt's official website http://www.frankfurt.de/sixcms/detail.php?id=2805&_ffmpar%5B_id_inhalt%5D=32019
Address (Unnamed Road), 60598, Germany
Coordinates 50°5'12.588" N 8°40'4.228" E