Stick with the Cheviots, they are rounded and if you poke around the edges you are always within reach of society if things go amiss. Just make sure you are above the farm walls which surround the area.
Try Hedgehope Hill on a summers day. You can pretty much walk in and out in about 4.5 hours with an eight year old. Meaning that if the tent blows down (which it won't) or the boogie man comes to get you (which it won't) you only have an hour or so to walk back to civilization. But there is no need to rush take your time and let the kid do the route finding with the map, abeit under your watchful eye.
Hedgehope Hill also has the advantage of some rocky outcrops, I think from memeory called the Cunyun Crags.
Although you may think that you should keep kids to the grassy stuff, kids love to get hands on rock and think they are climbing Everest. Grassy slopes bores them and knackers them out. Obviously don't let them stray onto steep rocks more than 10 feet in height and make sure you are behind them to catch them (which you won't have to, there like mountain goats) and keep a close eye.
You will have a good view, weather permitting of the main range of hills. Camp just off the summit and you will fire the kids imagination for their entire life.
The only problem with the Cheviots is a dependable supply of water. But, one night should not present a real problem to carry in what you need.
Be prepared to carry in food that they like, i.e tins of sausage and beans, and not lightweight stuff which they will probably not eat. Hotcross buns wrapped in foil on a stove with some butter in a small tupperware is good. Marshmellows over a gass flames tastes dirty!
Above all don't nag them and keep a large supply of sweets at hand.
Kids are born rough, it's only society that smoothes the edges.
