I guess I have a little bit of a different take on it. I'm not opposed to doing some things separate but I wouldn't want to spend lots of time apart since part of the point of doing Disney together is doing Disney *together*! So I'd look at it as just another aspect of family life where compromises have to be made - perhaps everyone picks their top 2 or top 3 "must-do" items for each park, and you all do your best to accommodate those items, and anything else is just 'extra fun'. If someone is not tall enough, then go for the Rider Swap pass. Sure, your 12 year old & 8 year old might roll their eyes a bit at getting a photo with Rapunzel while your daughter is star-struck, but that's just what families do sometimes. And then she'll maybe have to wait a bit while the two older ones ride a bigger ride - it's a trade off. But it teaches everyone that it's not always about your own happiness - it's about seeing the people you love be happy too.
I'm not suggesting you never separate, but I am saying don't feel guilty or stressed that everyone isn't fully entertained every second of the day. As a parent, I get a lot of joy from watching my kids experience things. For me, personally, I wouldn't want to spend my vacation mostly with my daughter while my husband spent his time with our son, because not only would we be missing out on family time together, but I'd be missing out on seeing my son's reactions, and my husband would be missing out on seeing our daughter's excitement. And, in the bigger picture of life, there's really not that many years for family vacations anyway.