I took my 10 and 5 year old daughters last September and, in addition to the attractions Damo mentioned, they really loved Jurassic Park River Adventure and Ripsaw Falls (rode it several times in a row). We also had a surprisingly good time in the Curious George play area (there is a place to shoot balls at each other). We are big fans of Camp Jurassic and like to spend time exploring the area. Pteranodon Flyers is one of our favorite rides, but your twelve year old is probably too tall. Your seven year old can probably ride and you can ride with him (you have to be under 54" or with a child under that height to ride).
We had fun eating in the Simpsons area, too.
My kids LOVED the pool at PBR and at RP. They both have sand play areas, but if I recall correctly, there is no slide at RP. You can pool hop, so if you have a lot of down time, you could pick a resort pool and try it out- even if you are not staying there. I would schedule down time at the pool if I were you. The pools are awesome. We had so much fun going to the pool first thing in the morning because there was practically no one there. Most guests head to the parks in the morning. We had no need since we had the free express passes and could go any time of the day and not wait in long lines.
My advice:
Stay as long as you can. We stayed seven nights and it was not too much because we enjoyed lots of relaxing down time and a few hours every day in the parks.
If you have to have a shorter trip, plan at least three park days. There is a lot to see and do.
Stay in a deluxe on site, if you can afford it. The express passes are great- especially with impatient kids.
Plan some pool time. The pools are fun and beautiful.
Study maps before you go and get the Universal app on your phone. It's great to have a plan and know where you are going.
Read about the restaurants and find a few you would like to try when you are there. We have eaten at several and the only one I would say stay away from is the Comic Book Cafe. It was not good. At all.
The water rides like Popeye, Ripsaw, and Jurassic Park get you soaked. Plan a few hours to just ride them, or ride them at the end of the day, or wear shoes you don't mind walking around in wet. They are conveniently close to each other. One evening we dressed specifically for water rides and just rode them.
Bring a waterproof lanyard. It was great for storing my phone, license, credit card, tickets, and hotel key cards.
Have a great time!