Another vote for considering a pop-up. We started off in a tent, but when our youngest developed an unhealthy fixation on unzipping the tent in the middle of the night, we moved up to a PUP with a lockable door. It was a great match for us. Our unit was very basic, just beds on wheels. I always hooked up the stove outside so we didn't have to smell food odors all day and all night. It also kept things cleaner. We did not have a potty in the PUP--indeed, my kids (who were about 5-7yo) would have been horrified to use the potty behind a flimsy curtain. We all preferred to go to the bathhouse for toileting and showers, but you may decide to put in a little porta-potty while you have very small children.
We really loved that little PUP. It was less expensive and it helped us figure out whether we even enjoyed that kind of camping without investing a ton of money. Later, we moved up to a trailer, but it was perfect for our needs when the kids were younger.
ETA: I never really worried about someone stealing the kids from the pop-up or the tent. I don't think I have ever heard of someone stealing kids from a PUP or tent. Not even from a guy who knew a guy whose girlfriend heard about it at the hair salon. :C
