Hi Josee! So excited for you...and you are so wise to start planning now!
There is easily enough to keep you going for a full week at Walt Disney World. I would start by thinking of a few 'must see/do' items to put on your itinerary. There are great Disney guide books available at the library, or check out www.disneyworld.com under the tab 'Things to do'. For a first trip, I would suggest at least one
character meal. Pick your children's favorite character and then find a breakfast or dinner where you can meet them.
About 6 months from your arrival, you will find the park entertainment schedules online. I would schedule the parades and fireworks into your itinerary right away, and then plan your days around those events. April can be very hot and busy at the parks, so we like to plan at least one or two afternoons just lazin' by the resort pool (whichever resort you are at will have an amazing pool!)
Your 10 and 12 year old will have no problem being tall enough for all rides; your little one may have to skip the roller coasters. But no problem, there are SO many things you can all do as a family. We LOVE the musical shows and the 3-D shows at each of the parks; they are suitable and fun for all ages.
Josee, I don't have much more room here, so write me back with any specific questions. Here's my last key piece of advice: To ensure you get any character meal or restaurant reservations that are on your 'must do' list, call exactly 180 days from the date you want the reservation. If you call 1-407-WDISNEY, they can tell you the exact date you can call. For an April trip, it will be this October sometime.
You'll all be stampin' up the steps to the Magic Kingdom in no time! Have an amazing, pixie-dusted vacation!