OK, so now I'm in Minnesota where my parents have been taking care of my kids for two weeks during our trip to Italy.
Upon arriving in Minnesota, I was a little jet-lagged and not quite in line with the local schedule. So I warmed up some leftovers for a solo meal (while chatting with family in the kitchen), and just as I was about to eat, my 8-year-old Nico suddenly announced that he was going to say the prayer!
Naturally, I was a little shocked (this is what happens when the atheist family leaves their kids in a religious household for two weeks?), but I didn't object or try to stop him.
Nico started by explaining that everyone has to be quiet until he says the magic word: "Amen!" Then he began his prayer:
"Our Heavenly Nico..."
He then said a few things about the food and about being thankful that Mommy could come. Then he gave a bit of a pause (as if waiting for a drumroll) and gave an enthusiastic "Amen!"
It was kind of a cute approximation of a standard Mormon prayer. It had the same basic structure, but he didn't quite use any of the standard phrases. Notably, he didn't close "in the name of Jesus Christ."
Later my mom took me aside to explain that they didn't tell him that he needs to pray or anything like that -- he was just spontaneously imitating what they do at their house. I replied that it's totally obvious that they hadn't actively taught him to pray. If you teach a kid about prayer, the one thing he's not going to mess up is who the prayer is addressed to!!
It seems pretty clear that it was just a question of "when in Rome..." Nico -- being a curious kid -- wanted to try out the strange customs of this new household. I imagine he'd heard my dad giving a prayer (starting with "Our Heavenly Father") and cleverly guessed that that was the spot where you insert your own role. (OK, he was wrong, but it was an interesting guess nonetheless.)
And, ultimately, I want my kids to learn that different households and different people do things differently; that the way Mommy and Daddy do things is not the only way. And I think we're on track. We'll have plenty of opportunity to discuss "What is prayer?" with Nico and Leo as they grow and have more experiences, and it's great for them to discover things on their own.