Our local (the few there are) charge a higher rate for their 'kids free' camping compared to the lower cost campgrounds that charge extra but wind up the same cost in the end.
(When we camp "local" we tend to have one or more of our grandkids with us)
Although this topic may be a WHOLE 'nother thread...
we are one of the fortunate *few* who have a 'membership' resort campground "local" to us. By the time we pay for eight nights at the local "nice" cg with free kids/pets or 12 nights at the state park, it's paid for our membership fee's for the year. Camping, depending if we are local or traveling, runs around $10 a night, but can be limited to one-three weeks for a stay then a week before you can return to the same cg. If by chance you also have an opportunity to join a "membership" campground that you actually *enjoy* camping at, make sure you purchase a "used" contract. The one we go to affords activities, but we can hike/bike/etc also (win-win for granparents anyhow). PM me if you want more info (maybe I should create a thread for this topic?...Hmmm)