My son is 6 and is very thin. We mostly put it on his bottom because that's where it stays on him the best. We use upper arms when we have to, but even when I really tape it down, it seems to get itchy and he inevitably ends up scratching it and pulling on it so much that the sensor fails early. Last week we tried it on his abdomen (at his request), but he had the same issue as with his arms. I think it's because the new sensor has such a high profile that it gets knocked around a lot more.
Mom to E, 5 years old, dx'd 3/11, pumping with the Ping since 6/11 (Novolog), Dexcom 7 (now G4) since 5/12
L, 2 years old, non-D
C, 9 months old, non-D