Is it really important to stay on property once for the kids first trip, or do we stay where we know?
Some people think it's important to stay on property
every time -- other people think you should stay on site once "so you know you're not missing anything and are actually getting more staying off site."

So the real question is, how important is it to
you?
Personally, I wouldn't sweat it for the kids first trip at all. Even if you do decide staying on site is important, it sounds like you'll be going again, so I'd wait. Especially if your kids are grade-school age or younger, I think they'd be happier with somewhere to retreat to that's familiar. They could have the excitement of the new experience, but also somewhere that wasn't stressful, if you see what I mean.
Depends on the kids, of course, but Disney can be pretty overwhelming on the one hand, and on the other hand, if you're going to go more than once, it's nice to keep some stuff back that'll be new another time.

The parks are the same where ever you're staying. Your kids will most likely have a great time where ever you're staying. You know you like Vistana, planning your first trip can be overwhelming, so just stick with what you know on that front, is my advice.
And to be honest, in your place, I wouldn't stay on site unless there was a particular resort I wanted to stay at. I'm not going to leave a place I love unless I have reason to think I might like the other place just as much. If you go to the Wilderness Resort for dinner or just looking around, and think the place is the coolest thing ever,
then think about staying there next time. Or if your kids start doing the, "All my friends stay there, why don't we?" thing. Maybe.

But I wouldn't leave something I loved without a good reason.