Thanks for your answer!
As a foillowup question to http://dinonline.org/2017/07/11/swimming-on-17-of-tammuz/ What about children are they allowed to swim today?
Regarding children on 17 of Tammuz, although they don’t fast, however from when they reach the age that they can understand the concept of mourning, and should be taught to eat only basic foods on a taanis and not delicacies. Regarding children swimming on the fast day, it would be the same as during the nine days that from the age of chinuch they should not go swimming.
M:B 550-5, Aruch Hashulchan 554-9, Beer Moshe 8-95 (7). Regarding swimming in the nine days, this is the opinion of Horav Sheinberg zt”l and Dayan Roth shlit”a, brought in Children in Halacha pg. 188.