Interested in the response too. We'll be there next summer with our 3 boys who will be 15, just 14, and 12. DH & I would like to do the Keys to the Kingdom tour and I read on another post about parents doing this tour and letting the kids just do MK at the same time. Might be kind of cool for my kids to have some time alone and do their own thing. We were considering either this tour or the Segway tour at Epcot, either of which none of them are old enough to do.
My first concern is what if we're in the tour and something happens. Seems a little more challenging than just splitting up, but I guess as long as we all have cell phones and we're in the same park, it shouldn't be too bad. Maybe my concern is losing the money if we have to interrupt the tour because loving DS's lost their little brother!
Second (and this is just me since I'm obsessive!) my concern is how much time they would waste. If this was at opening, I would want them using all the DIS/TGM/UG strategies to do all the big rides they want while getting FP's for others to make the most of their time. Unfortunately, I think their strategy would be along the lines of going on Buzz Lightyear 5 times in a row and then playing in the arcade or going in the gift shops or eating snacks somewhere! Hard for me to let go and not micromanage their time too!
I think with your (and my) kids' ages, and the fact that there are two of them, nothing major should happen. As long as one has a cell phone, either could use it if anything did happen (I'm thinking more along the lines of "I don't feel well", "I lost my brother", "Can we leave now?"). In an actual emergency, I feel confident that CM's would help them and one could still contact you.
I guess I would strongly have to remind them about being polite. I can't imagine them bothering little kids, but could see them messing around in line and bumping people or being loud ("Stop!", "No, YOU stop! I was here first!"). I have to believe they only act this way when we're around to referee and would be better on their own, but I guess you never know. Can't hurt to reinforce ground rules!
When are you going? We don't go until late July. If you go before then, please let us know how it goes!