Welcome!!
The best advice that many of us would give you is the same: gather lots of information. Read the Unofficial Guide to WDW and get a feel for what sounds interesting to you. They have plans that you can follow for 1 or 2-day trips.
If the hopper option is not out of reach, do it. You'll be able to get a taste of more of the parks. If you can get your gang off to an early start, you should be able to hit many of the major attractions. There are lots of attractions I could list that I would skip if I only had 2 days, but they'll be different for many people.
Will it just be you with the 2 kids? In asking that, I'm assuming that your dh(?) is there on business. With a 5 and 12 yo, you'll have to do some compromising if you're on your own with them. I'm assuming your kids will want to do things that the other doesn't want to or can't do (height??).
Have fun planning. Keep reading and asking questions.