Credence Barebone has been kept sheltered from many things. When he meets "Mr. Graves," he finally has someone who he can ask questions to. Some of these are not questions one usually asks...well, anyone really.
Ex. "Mr. Graves, what's heavy-petting?" "Someone said Ma was suffering from hysteria and needed a vibrator. What's that?"

How Grindelwald reacts is up to the filler.