Easterhay said:

What could I do but shrug and say - once again - "They messed up."

You could have been a little more explicit, such as: "They weren't very careful with continuity problems when they made the prequels." And you could even explain what continuity is, how there's people whose job is to maintain continuity, and how there was actually no continuity problem with this scene until they made the prequels.

But yeah, they messed up a lot of things in the prequels, not just continuity. Oh well, your kid's pretty astute to pick up on things like that.