In general, there isn't a whole lot to putting together an itinerary (it's not like Disney-intensive), and the EPs will be INVALUABLE during your stay as far as reducing waits and helping you to see everything. I'd start with whichever park has early entry, and hit the Harry Potter attractions on that side of the park. I think the only rides your youngest won't be able to go on are Rip Ride Rocket and Revenge of the Mummy in US, and Incredible Hulk Coaster, Dr. Doom's fearfall, Forbidden Journey (Harry potter), and Dragon Challenge (Harry potter) in IOA.
Depending upon how flexible your youngest is, you may be able to do child swap with the 16 y/o so that she can go on the bigger thrill rides. If your youngest won't be too upset by that, that is. For us, we have a family rule (easier, since we only have 1 child), that we won't go on any ride that kiddo isn't big enough for (being a short family, back in 2012 when we last went, DS was just over 48"). So we didn't do any of the thrillier rides.
The thing I like about Universal, especially with the express passes, is that you don't really have to worry about intensive itinerary planning. Once you do the Harry Potter lands, you're free to hit the rest of the parks as you'd like. IF you're doing early entry every morning, you could probably do US one day (starting with HP), IOA another (starting with HP), and then on your last day, hit your favorites again, using the train to commute to the other park...
There are a couple good planning sites too. I like Orlandoinformer.com for park info, and touringplans.com for their touring plans. The latter is a paid subscription (I think $7 for a year of access to their plans and customization for the Universal part of the site), but well worth the $$ considering the $$ you're dropping on vacation...it's the price of a butterbeer, to put it in perspective!