The travel times to MK and EP are about the same from each of those resorts, give or take a few minutes. Generally, it takes about twenty minutes to reach either MK or EP (although your trip will likely be shorter--this is on the far end).
CSR and POR have the advantage of multiple bus stops--no matter where you are in the resort, you likely have a bus stop that's not too far away. There is only one stop for all buses at POP, in front of Classic Hall. POP also has a slight disadvantage as it's bus stops tend to be the farthest away at the parks (due to the fact it's a value resort).
Another thing to consider is whether or not you enjoy spending time at your resort--CSR and POR both have excellent pools, as well as many other activities (biking, fishing, and such). POP has multiple pools, but they lack the slides and fancy features of the moderate resorts. If you tend to be park commandos, this is not a concern.
Considering the age of your kids, however, POP or POR would probably be the most appealing--both are very well themed and have a lot to see. You'll enjoy any of them!