I can see no problem in being open. If you are sure he knows the info comes from you, so what?
If a warrant is executed on his property the evidence for that warrant will come from the police (via you).
If child porn is found in his possession, I doubt very much your testimony will be needed if the matter goes to court. Possession is pretty much banged to rights, especially if he lives alone.
If he receives a caution, then obviously there will be no court.
But to be honest, if you have kids, I really do not see why you have even posted this question. I am childless, yet would not even think twice about taking a course of action.