We had a great trip, there and back. And I was surprised to be greeted by name after mooring in Two Harbors. It was jeepbluetj who was as helpful and gracious in person as he is online. Like he mentions, there was very little wind early in the day. I almost pulled the plug when the forecasts started heading in that direction. But in the end we went, motoring until we had wind. One nice thing about a 3.5-hp outboard is that you get great gas mileage. We ended up using the engine (just about) the whole way back to King Harbor today. I think we used a gallon and a half of gas for both directions.
The kids had a great time. They both want to go back sooner than next year.
We had a tiny inflatable that I'd brought on the far-flung chance that the boat ran into trouble and a random flotation device might come in handy. But we inflated it just before arrival and I announced that their challenge was to get all of our gear to shore with it -- including floating the captain himself. They were itching to go swimming anyway and had a great time going back and forth with the inflatable between the string line and the beach.
Here's a video put together by an app on my phone. I've got a lot of other footage, which I'm sure I'll turn into something. But this clip shows our route and some random snapshots from the journey.
It would be nice if I could say that we proved we were capable of making the crossing. But there weren't any particularly-difficult challenges that came up on this particular trip -- nothing that soundly proves our skills as sailors. I'll be at least as cautious the next time we try it. It won't always be this easy.
But the help from this discussion was really significant. Just the mention of the Isthmus Fan removed what would have been a perplexing weather phenomenon from two miles out. Because of this thread, it was easy to explain to the kids and we managed to navigate it, aware of one option of how to move around it.
So thank you, to everyone who replied here. My kids love the island.