Child age 5 kick to testicles, pain gone in 5 mins, no swelling/bruising/bleeding. Can you confirm this will not reduce sperm production as adult?

Very unlikely. If there was no injury to the testicles; this is not something to worry about. All the best.
No...but unlikely. The adage never say never applies in this case.Males routinely experience simple trauma to their scrotum/testis that varies in intensity.Testes are surrounded by fluid & tissue layers that absorb some of the trauma.They can lose the function of one & still be fertile. In more than 3 decades as a pediatrician I have never seen or read of a kid losinge one or both of their testis from such trauma.