Campfires, campfires, campfires. The best thing about camping and sadly not nearly as common as it should be. Like someone else said, selling wood is a good extra moneymaker.
Family bathrooms/showers are a great idea with mixer taps that you can turn on and off. I just spent the weekend trying to hold my toddler up to the basin, press the cold tap (as hot water scorching) and get her hands underneath it in the 2 seconds the water stayed on - resorted to wipes in the end.
Free clean reasonably powerful showers, loads of hooks and maybe a bench to put stuff on in the cubicle - space permitting. Regularly topped up loo paper (pleeeease - I always forget to bring mine and have to go back for it).
Level pitches where possible.
No need for pools/clubs/bars etc but a good shop with camping essentials (tent pegs, gas bottles, head torches, waterproofs) a good idea, bread and milk and maybe some booze too. Lots of locally sourced food not a bad idea either and the option for ordering bread (and even papers) for the morning is a good one too - payment on ordering obviously or could be a financial loser. We stayed on one site where a van turned up in the middle of the site every morning and you could buy bread, bacon, doughnuts, papers and other camping essentials there - very handy.
Lots of space for children to run around in and explore and maybe a small playground for the little ones, though that is not essential by any means.
Maybe a family camping area separate from groups of single people if that is an option and you want to encourage both.
If you have the space and inclination, a barn or covered area of some description would be fantastic for cooking and eating when it is raining. This is less for the super-organised family camper with their cooking tents and awnings everywhere obviously but great for those with small tents and less facilities. We stayed in a site in Spain which had one of these and found it a godsend when the heavens opened one day. Good place for socialising too. One place in Wales made bacon sarnies and tea and coffee at breakfast time in their barn and provided tables and chairs to eat them at. Very nice and, again, not a bad money-spinner particularly in the rain.