"Son/Sweetie, do you know how babies are made?"
1. If they (think they) do...
"Yes, dad. I do."
"Oh, really? Tell me what you've learned."
1a. If the information they learned is correct...
"I'm glad to know you've learned the correct stuff about love." ...and then the talk would end there.
1b. If they were taught something that was completely wrong...
"Not one word is correct about that. Now, let me tell you how babies are really made..."
1c. If some information they learned had flaws...
"Actually, let me correct you on some things..."
2. If they didn't know...
"No. Are you gonna tell me?"
"Well, now that you're old enough, I think you're ready to handle this stuff..."