Hi Stefanie!
I think you'll have plenty to do with both kids at Walt Disney World! In fact, with it being just you and your two children, it's the perfect opportunity to really enjoy all the magic Disney has to offer, by slowing down and exploring the parks at your childrens' pace.
For starters, I think you could spend a whole day at Magic Kingdom, and maybe still not get through everything. Or even better, go on the kids favorite attractions twice, or more! My sons have almost the same age difference as your children, and I can tell you that they both enjoy the same attractions at Walt Disney World. Sure, the older one might earn a higher score on Buzz Lightyear's Space Ranger Spin, but the little one is happy to go on it over and over again just the same! Also, don't be surprised if you find that your younger child shows favoritism towards a particular attraction...I have a hard time pulling mine out of the car on the Tomorrowland Speedway once our turn is up!
You can repeat this experience at any of the four main Walt Disney World theme parks, really. If it helps, there is a great page
HERE, and on there you can find park maps with attractions for little ones highlighted, and even height requirements for all the attractions can be found on a pdf on the right hand side of the page. Anything not listed on the pdf can be experienced by guests of any height.
I hope this helps assure you that you can definitely have a fun time with your children. I'll be visitng in April, too, so if you see a Mom in pink wrestling her toddler out of the car on the Speedway, be sure to say hi!
Have a magical vacation!