If you map it out on a mapping website, depending on the location of the Little Mermaid room you will be somewhere between about 450-650 yards from the bus stops (so roughly 1/4-1/3 of a mile). The average person can walk that in less than 10 minutes. The furthest room at ASMo would be about 450 yards. The longest trek at Pop would be about 400 yards. Some of those distances can be cut down by cutting through parking lots or other shortcuts. In the scheme of all the walking you'll do during the day, the rooms are a very short distance from the bus stops. However, I know that at the end of a long day in the parks, those distances can feel like miles. It really just depends on how you think your family will handle it. Obviously it doesn't bother a lot of people because the Little Mermaid rooms are always sold out. AoA is a great resort and I think your kids would probably love walking through it each day. But keep in mind that if you stay at Pop, you can still walk over to AoA just to look around and/or have a meal. You could even take the bus back to AoA from a park one day and just spend some time walking around before crossing the bridge to Pop.
FWIW, we stayed at Pop several times when our kids were young and they loved it there. They recognized plenty of the icons. All they really cared about was that we were at Disney World and there was a pool.