We went in April last year, the queues were no where near as bad as we expected. My dd's were 6 and 4 at the time and loved every minute. It was the best holiday we have had and we are now saving so we can go back within the next couple of years.
We bought our theme park tickets before we went. The disney tickets gave us unlimited access to all parks for 14 days, they were well worth it as we spent most of our time at the disney parks. We also bought another ticket which gave us unlimited access to both universal parks, busch gardens, sea world and wet and wild. When we go again I don't think we would get this ticket as we didn't really use it that much.
Universal studios - was good, we went twice as it was so close to our hotel, but I think it could be done in a day.
Universal Islands of Adventure - wasn't really suitable for young children, most of the rides have height restrictions and there was only 2 rides dd2 could go on.
Busch Gardens - about an hours drive from Orlando, but worth a visit if you have a car.
Sea world - was okay, but not as great as we expected it to be, we only stayed about half a day.
IMO the disney parks are best for young kids. When planning our holiday before we went, we underestimated the amount of time we would spend in the parks, We though we would do magic kingdom park in a day, but ended up going about 5 times as there was loads to do.
One of the best things we did was have dinner at Cinderella's royal table, it is at the top of the castle and really was magical, if you want to do that or any of the character dining in the parks, book before you go. We waited until we got there and were really lucky that they had a cancellation.
Also, there is lots of walking, dd2 was 4 and at home never used a buggy so we didn't take one. The first day in the magic kingdom park we ended up hiring one, but it cost about $10. The next day we bought a fold-up buggy on International drive for $25.
We stayed on International Drive which was nice but is about 20 mins drive from the disney parks. When we go again we are hoping to stay in a disney hotel.