I like the alarm idea myself. In the event of an emergency (such as a fire), I would want the five year old to be able to escape--and, in fact, it would not be a bad idea to show him how the door latches and locks operate. Recall that one generally has a lot less time to get out of an RV in the event of a fire than one has in a "typical" sticks-and-bricks house fire, if there is such a thing as a typical fire.
If you do go with a deadbolt keyed from both sides, I would suggest having a key for it available someplace outside or on your person as you sleep. That way, if you had to get out using a different emergency exit, you could open the door from the outside to let/get the children out.