It definitely gets better! We camp for a week in the New Forest every year, have done for the last 19.
The years before children were fabulous, when we had one who was just 6 weeks old, it was easy enough, the next year we had a one year old and I was in the first trimester and about a 5 minute walk from the toilet block, so less fabulous.
The following year, we had a 4 month old (easy) and a 2 year old (needing constant entertainment), so it was busy, but still fun.
Ages 1 & 3 - busy.
Ages 2 & 4, starting to become a little bit like hard work.
Ages 3 & 5 to 7 & 9 - miserable. For all concerned.
Ages 8 & 10 - 70% fun, 30% Whinge Central.
9 & 11, completely wonderful! So good in fact, that we stayed an extra 3 days! They could go off on their bikes for hours at a time, the eldest had a phone with a tracker so we knew where they were and they could call us in an emergency, plus we could arrange for them to be back at the tent at a certain time. swing ball kept them busy for hours, they read, played cards, made friends with other kids and generally left us to read and chat - I can't wait till next year! Of course, it's only 2 years till we'll have a teenager so perhaps 2018 and 2019 will go down in our family history as "The 2 years when we all enjoyed our family camping holiday"...
Holidays with whingey kids can be soul destroying, I agree with PPs - take them where there are other kids they can play with and take it in turns with other parents to watch over them.