You can place as many structures as you need as long as they "fit" on the site. They must be contained within the pad and driveway.
We had a little utility trailer one time that we tried to park beside the driveway to get it out of the way and got a note on my windshield.
I then pulled it up to the extra parking lot and locked it up for the week cause it just took up too much space.
I don't put a tarp over my entire site because for me it is just too much work to put it up and make sure it is tight enough for water to flow off of it instead of collecting water.
I put a shelter in front of my tent so that I have a dry place to shuck my shoes, rainsuit, and a place to sit in my chair. It keeps my tent drier and cleaner.
I just make sure I waterproof my tent thoroughly.
I definitely agree on an extra tent for your kids. Even though those family tents say they sleep 8, they are really only good for 2-3 people.
Those smaller tents that say they sleep 4 are really only good for 1 person.
My recommendation would be the larger tent, a fly in front of it, the smaller tent for the boys, a fly in front of it, and then a screen house over the picnic table in the driveway.
Carol