I have been to 3 haven sites since having DS now 9. They tend to be well positioned with lots to do nearby and most are close to beaches. They have lots of entertainment especially for younger kids which you can take or leave. I avoid quiz nights and bingo which I hate :) While my DH wouldn't choose a Haven holiday for himself in a million years, he was happy to go for DS after we learned out lesson staying term time at a boutique hotel abroad when DS was 3. Lovely for us but nothing much nearby and no kids for DS to play with hence less restful for us!
Much as I will happily go to caravan parks, they are not for everyone, I know what your sister means. Haven is usually a good mix of 'normal' families and loud families who are a bit 'rough'. I have the impression that generally some sites are much nicer than others and others it is just a bit of luck who goes at the same time as you. I imagine Parkdean and Pontins to be worse!
Greenacres in Wales was great and not that big. We went out of season before DS started school which is a different experience to school holidays! Entertainment was fun. We were back in the van by 9pm every night as DS wasn't very old. The place wasn't overrun with families with being term time but there were enough about for DS to play with and and busier at the weekends.
We went to Cornwall on a last minute deal first week of school holidays one year and only paid £500 which was a big saving on the regular price. That site was lovely and the entertainment area big enough to cope with the numbers. The entertainers were fantastic and DS had a ball. We had no issues with the clientele.
Littlesea at Weymouth was a different experience also in the summer holidays. The site was fine and but the pool areas small (they were restricting numbers on the occasions we tried to go) and the entertainment area also struggled to cope with the numbers. There were big queues at the bar and people rammed in so when DS was down the front for the kids entertainment, we couldn't really see him properly. It was also all a bit much for him so he ended up being sat with us and not interacting and joining in as much as he had on previous holidays which was a shame. This was the only site where we came across larger numbers of customers being loud/shouting/swearing. Some of this was on one particular night and caused by the lack of food available at the end of the night - takeaways struggling to cope with demand, over an hours wait for a pizza, lots of grown ups had a few beers, overtired hungry grumpy kids - you get the idea. We camped 4 nights on that trip and were on the over spill campsite where we paid a good price. Our field was lovely and spacious with nice people camping nearby and lovely staff but the electric pitches in the next field were really on top of each other and a lot of the kids and parents on the park nearby were loud/abrupt/shouty and not particularly friendly.
I would go back to the Wales or Cornwall sites but not to Littlesea and would happily try some of the others.
For those saying is Center Parcs really that different, I would say yes it is. Absolutely. DS and I have been twice recently and had a lovely time. The staff were really friendly and helpful. 99% of people are smiling and seem relaxed interacting with their kids and having proper conversations with them rather than being fraught and shouty which you do see quite a bit at Haven. Even on changeover day at lunchtime in the sports bar, despite the place being rammed with people most if not all seemed to be taking it in their stride waiting for tables and food with minimal fuss. Makes for a generally more pleasant experience for everyone around. It all feels very safe in my experience. We have booked again for next year as with just the two of us we can stay in one of the hotels which brings the price down considerably (no catering facilities just fridge and kettle). Definitely worth a look for any single parents with only one child (or those leaving the OH at home).