Former Disney resort CM chiming in:
Yes, unfortunately guests sometimes leave their common sense at home. This happened most often while they were around the resort (running to the food courts or the pool). The bigger problem is assuming your door closed and locked behind you- ALWAYS double check the handle! Something as simple as a shoe or one of those stupid pizza flyers could get in the way of the door closing completely and suddenly your possessions are unguarded.
As for the issue of kids losing their room keys, there are a few things worth mentioning:
1) At least as of 2010, all kids room keys were automatically disabled from charging. The CM would have to manually check a box to authorize charging.
2) CMs are required to ask not only if you want charging on the room, but which members of the party are allowed to charge. It was rare for guests to authorize their kids.
3) In the 2 years I worked front desk, and the 5 years my friend has, neither one of us EVER saw a KTTW be used fraudulently. Guests lost their KTTW daily, but we never had a case of mysterious charges. The KTTW is immediately cancelled as soon as it's reported stolen so there are no further risks.
With all of this said, room break-ins were very rare as a whole. As long as you make sure your door is locked every time you leave, there's practically no chance anyone gets in. If any hotel staff were to enter your room, the lock monitors who came in and at what time. I never heard of housekeeping stealing anything either- with the exception of the hotel's toilet paper

