A dog put my child’s tennis ball in the mouth, how do I make the tennis ball pure?

In the name of Allah, the most Beneficent, the most Merciful.

Answer  

According to the Hanafi Fiqh, the body of a dog is not impure, rather its saliva is.  (Raddul Muhtar p.382 v.1)

With regards to your question, as the saliva touched the ball, it will be necessary to remove it.   One should wash the ball until the body of the impurity is removed.  (Nurul Idah p.59)

Only Allah Knows Best

Mohammed Tosir Miah

Darul Ifta Birmingham