Personally, with that many people, I wouldn't even try to spend whole days together. It's difficult enough accommodating the interests of a small group. With 25, that sounds miserable. I'd plan for a meal and maybe a parade or show throughout the day together, but then let each individual family unit plan their own rest of the day. Have them make their own fast passes, because in all likelihood not everybody is going to want to do the same things at the same times, get to the parks at the same times, etc, and trying to move around the parks all day with 25 people is likely to be difficult. You might be lucky and be able to get FP for 25 people within a few mins of each other for certain rides, but for the popular ones, it will probably be much more difficult. As for same-day FP, again, you might luck out with stuff like the Little Mermaid or Nemo rides, but even that might be a stretch. If each family unit worries about their own FP, everyone is more likely to be able to do what they want and get same-day FP for their own little group for things they actually want to ride.
I strongly recommend that you don't take this all on yourself. I say this from personal experience when I was just planning a trip for my family unit and just 2 other family members. They insisted they were fine with whatever I wanted to do, wanted me to make all the plans, and gave zero input. They assured me they were just along to see the kids enjoy their first trip. I tried so hard to incorporate things that I thought they'd enjoy, even if it was stuff my family didn't really have an interest in. Overall, it didn't go so well. They didn't like how I planned each day, didn't want to get up in the mornings, we missed a couple of FP, an ADR (which cost ME $60 for the no-show fee), etc. They felt left out when my family of 4 went on rides all in the same row and they had to sit in front of or behind us because the kids wanted to sit with us while experiencing a ride for the first time. Overall, they felt like I was herding them like cattle through the parks, didn't understand why we had to go to a FP or ADR now when they'd rather do whatever else (but still wanted to do the FP and ADRs at another time), and I felt like all of my hard work that went into planning and trying to incorporate things they'd like went unappreciated.
While I am sure many families are much more go with the flow, it is still going to be hard to make that many people happy on a trip, and it would be a huge undertaking to try to plan it all.