But if you'd folded the buggy, wouldn't the kids have still been in the way as you tried to go down the aisle?
It's a narrow aisle, so there's really not room for two full-sized people to pass easily, is there? And you *and* your DH would have been carrying something semi-bulky (you, the kid and any tote/diaper bags; him, the buggy and any other tote he had). So the kids would need to get out of the aisle.
Plus, the aisle is for walking, not for standing or playing, especially during loading and unloading times.
I don't think you were rude in the least to ask them to move. And I think it was fine for you to say, "could you sit down" or "could you please step out of the aisle." I think w/ kids, often "excuse me" is not actually clear enough. They don't think as far, they're still in training. So you tell them what you want them to *do* (or, better yet, *ask* them). I think you were fine to first say "excuse me," since at 7, they might have been able to figure out what you needed. When they didn't get it, then you were fine to ask them specifically to do [x action here] that got you what you needed.
So, minor goof on the buggy thing. Nothing wrong at all in asking the kids to please move out of your way.