I’m in the release order camp, love the amount of consideration into the question! Honestly the best thing you wrote was letting the kids go through it at their own pace and not try to marathon them. Considering the ages I think now is the right point to take your time letting them soak in the originals, uninterrupted, they won’t be able to appreciate the prequels in the middle of the OT. I would save those for maybe three years older after they are already invested, then I would be prepared for the oldest to even prefer the prequels for a brief time. But at a certain point I would just feed them whatever Star Wars they wanted, if they’re brought up with the OT I think that will resonate the most.