Well moderated topic. Well done. That said...
IMHO, It really doesn't matter if it's a clean and well lit Walmart lot, or a secured CG. In comparison to a ghetto Walmart or a free county camping site out in the woods somewhere. It's all the same to me. The key is if you don't feel comfortable spending even one night anywhere, and you have a choice, don't do it. Those accustom to boondocking would be more prepared for a stay in a parking lot or rest area. Others, including newbies, may not be so prepared. Planning is the key. Experience is the best teacher.
As far as personal safety for you and your family, it still doesn't matter. Regardless of where you lay your head down for the night.
Bluntly put, one can get mugged, robbed, or shot in any Walmart lot. As well as any campground. How much do you want to worry about it?
My point is our place is out in the woods. My property borders over a 100 acres of national forest. Open to anyone. 100's of acres of private land surround our property on the other 3 sides. When we are not there, all bets are off. I could loose everything. I do have a neighbor who lives next to my place (approx. 150 yards away), but he can't babysit it! When we are there, a firearm is handy, but not relevant in this case.
If an unwelcome visitor becomes a problem, they are first greeted by our dog (German Shepard). No problem.
It really all depends on how comfortable/prepared you are, where you are, and how well you can handle a situation. A lot of common sense is also involved. I do give Walmart kudos for making their lots available to RVers, Most retail chains do not. In a pinch, you have the option.