There were two:
Does Jesus make people do stuff?
and
What does “crucified” mean?
My answer to the first question (after a long pause): “Jesus can make people do stuff. Jesus can do anything he wants to. But he usually doesn’t make people do stuff. He lets them decide.” Some will disagree with this. But I think it has pretty good Scriptural warrant. I’m sure this question will come up again. And I thought the sex question was hard!
The second question I answered as specifically and succinctly as I could. I actually got a little teary-eyed as I described crucifixion to him. His response to my answer was appropriate, I thought: “Why did they do that?”
Words on the Word 
My 7 year old recently asked the following (after watching episode IV, V and VI of Star Wars as a family): Can God forgive Darth Vader? Which prompted a discussion about evil and humanity’s freedom (?) to reject God, which made my 4 year old ask, “What is ultimate evil?” Ahh, dinnertime discussions!
Ha! That’s awesome. “Can God forgive Darth Vader?” sounds like a children’s theology book waiting to happen.