The age spread between my oldest and youngest is 12 years, so I hear you! Don't try to do too much, and realize that sometimes someone will be bored or tired or whatever. The guidebooks all suggest a midday break - if you can make it back to your hotel, your daughter can nap while your son gets pool time. (My kids always wish they had more time in the pool.)
For Princesses, look at your map and schedule (they hand it out as you walk in) and find out when story time with Belle is. You can also see and get photos with at least 4 princesses by standing in one line at the Fantasy Faire (just outside Toontown.) The line will be long, but your daughter will love it. Maybe your husband can take your son to Roger Rabbit's Car Toon Spin while you're in line - it's nearby, and not TOO little kiddie. ;) Also, Ariel's Grotto offers princess breakfasts. It's pricey, but you're guaranteed 1-on-1 time.
A lot of the "storytelling" rides - Pirates of the Caribbean, Peter Pan, Nemo Submarines, Soarin' Over California - are fun for all ages. California Adventure has some great all ages shows. Try the live Aladdin show, MuppetVision 3-D and the live show in front of the Monsters Inc ride - I can't recall the full name, but it's something like, "Drawn To The Magic." It's a fun live show, hardly ever crowded, and afterward you can get photos with Cinderella & Mickey.
The Redwood Creek Challenge Trail (in California Adventure) and Pirates Lair on Tom Sawyer's Island (Disneyland) offer great places to get out of the heat, play, explore and meet fun characters, with far fewer crowds and far shorter lines than most places. There are things for both your kids' ages.
Have fun! Don't try to see and do it all, just try to make a few great memories! Don't stress - just enjoy! :)
Updated
Something I didn't add yesterday - there are a few places where there's a toddler friendly spot next to something for older kids, so it might be good to split up, one parent with one child. For instance, in California Adventure, the 3-D show "It's Tough to Be A Bug" is too scary for your daughter (trust me), but your son will enjoy it. It's next to the Bug's Life fair area that the 2 year old will love, so you can take her there while your husband takes your son to the show, and meets you afterward. California Screamin', the big roller coaster, is next to King Triton's Carousel, so one parent can stay with the 2 year old while she rides (probably 2 or 3 times in a row) while the other goes on California Screamin' with the 12 year old. On the gondolas on the huge wheel with Mickey's face on it, one group can do the swinging gondolas while one does the stationary ones. You can be apart but close, and have more fun than insisting that everyone do every ride together. Have fun!