Remember that they can always visit Guest Services to add on more days. Perhaps they don't realize how much fun the parks are. The first time we went to WDW with my brother's family, they only bought 2 day passes because their kids "don't like rides." Yeah, they upgraded their tickets the first afternoon
As many others have said, everyone travels differently. We've had a couple of trips with large groups and here's what we do:
1 - plan for ourselves only
2 - let others know our plans and invite them along if they want
3 - make it VERY clear this is THEIR vacation and they are NOT required to join us, but are more then welcome to
4 - on a park day, everyone that wants to go to that park that day hits the first ride together, then we split up (you just can NOT keep 20+ people together all day without missing a lot of things and frustrating people)
5 - try to plan a meal out as a group ... we usually hit Medievil Times as they can accomodate large groups and this is often the highlight of the trip
6 - try to get together in evenings ... usually over margaritas! ... take this time to answer any questions people may have and just visit
7 - never get offended if you never see people ... even if you've covered their accomodation it is THEIR vacation
Also keep in mind that while the cousins may not get the chance to see everything, you're not their parents. This might sounds obvious, but I've found that differing parenting styles can be one of the biggest causes of friction amongst travellers. I know that my daughters and their cousins would love to spend every day together, but my brother and SIL go out of their way to make sure that doesn't happen. In fact, they've gone so far as to text that they're at AKL for lunch then we find out later that they were in DD ... I think they thought we were going to follow them. Odd, but that's family. You get what you get

We don't take offense at these actions, it's just something to chuckle about.
One last thing. You sound like you're a Disney veteran while your relatives are Disney "newbs". Be careful of your enthusiasm. I know that my first big group trip I came off as a bit overbearing, but I was just gushing with enthusiasm and excitement for people getting to experience one of my favourite places for the first time. I had to reign myself in and I apologised to everyone explaining I wasn't trying to take control over their vacation, I was just trying to provide them the best experience possible ... and that's where I went wrong, I should be "helping" them to have the best experience, not "provide" them with the best experience.