Agree with all the above. Although I am at the other end of the kid spectrum (my youngest just graduated high school) Our adventures started with day hikes, then evening/return to the car in the dark hikes, then did a play time "sleep out" with the tent in the front room.  Paper clips unfolded into an S shape hook into the carpet quite nicely.  The first time out "for real" bring some conversation starters for getting into the tent, questions like "favorite thing today" ice breaker style.  Plan on you not getting enough sleep, and be ok with that. Michael's suggestions above with the "something we never do" like marshmallows is just what we did.  Make a treat of being out there.  

The thing that didn't work so well for us was a schedule. It was tough going from home where things happened like a routine.  I think we interupted play too much our first couple times all in the name of avoiding a meltdown.

The first couple times I tried keeping them fairly clean.  That wasn't so helpful. We then changed our thought process, even helped them get dirty. Kids are curious beyond measure, and for the most part, nothing out there "hurts" them.  Help them explore. If they're out doing that all day - couple that with the little conversation in the tent and they will be snoozing without time to get scared.  We read to our girls most nights, as part of the bedtime routine, so bring along a favorite and read, to give a sense of "home".

You have such fun adventures ahead! (even if they are quite a bit of work ;o)  )