It's really tough to answer this one without knowing a bit about the makeup of your family and of course what you like to do, but there are a few "not to miss" items.
To really take advantage of the day, GET THERE EARLY, i.e. before the park opens. You will ride more rides in the first two hours and save a LOT of waiting in line.
You must take advantage of Fast Passes (FP). Get one as soon as you get into the park, and get another one EVERY time you can, usually if you get one as soon as the park opens, you'll be able to get your next one about 40 minutes later. After that they'll start being longer waits for the next one, but it's written on the ticket when you can get the next one. IMPORTANT!!! The time to return on the Fast Pass can be ignored. As long as you use it after the first time you can go back, you'll be fine. I suggest collecting a few in the morning to use in the afternoon when lines are much longer.
Start with Disneyland must do's:
Classic Rides: Jungel Cruise, Pirates, Small World and Haunted Mansion
The Mountains: Splash (FP), Space (FP), Big Thunder Railroad (FP) and Matterhorn
Cool experiences: Star Tours (FP), Tiki Room (do this when you're hot and tired and everyone needs a bit of rest), Train (again another ride designed to help you rest), Indiana Jones
So here's the thing, you're going to have to sacrifice many things if you're trying to do both parks in one day. IMHO parades will be out for you, but maybe if you don't mind having a distant view you can catch just the parade. Rides like Peter Pan, Pinnochio, Pooh, Alice in Wonderland, Mr. Toad, Fantasyland Boats, Dumbo, Tea Cups, pretty much all of Fantasyland are pretty slow loading rides and there are no fast passes. Lines for these rides can, and will get very long. If you want to do any of them, do them right after you get your first FP, and be prepared to only do only a couple of them.
California Adventure has fewer must do experiences, so you can pick and choose what you like.
Must do's IMO: Soarin' (FP), Tower of Terror (FP), California Screamin' (FP), It's a Bugs Life, Muppet Vision 3D, Midway Mania.
Midway Mania will be a long line and there are no Fast Passes, you'll need to decide if you want to ride it or not, it will likely take up to 60 minutes waiting in line.
Grab Fast Passes for whatever you can as quickly as you can.
Nightime entertainment, you'll need to decide between World of Colour and Fantasmic. In either case, expect to have poor viewing positions as you don't have enough time to stake out good spots for either show. You can get World of Colour Fast Passes, but that means going to California Adventure early which means you'll miss out on shorter lines at DL.
I highly suggest you take a good long look at all the experiences offered at both parks and decide what kind of rides you want to focus on, then create a plan of attack based on where they all are in the park, as well as where you want to be to grab your next FP. There is a great site
www.touringplans.com where you might be able to get some help, though I just looked and they don't have a one day DL / CA plan. Simply put, you'll be skipping a lot of things so only you can decide what can be skipped, and how long of a day you can keep.