I have 1, 3, and 5 year old daughters and we do visit each of the four theme parks. I recommend allotting 1 day each for Animal Kingdom and Hollywood Studios, and 2 days each for Epcot and Magic Kingdom. Epcot is a fantastic park, but it can be appreciated more by older children and adults, so if you need to trim the schedule I'd start there. Magic Kingdom will hold the most appeal for your preschoolers.
Park Hopper tickets can be great. However, many families with young children find that they don't use the park hoppers because it's a hassle to switch parks with little ones and strollers. If you're looking to save a few dollars in your budget, this is one of the first things I recommend you trim; you can have a great vacation without hopper passes.
ALL Disney hotels are child friendly, so make your selection based on the themeing and price that appeals to your family. We recently stayed at the Grand Floridian, Disney's flagship resort, and there are little kids everywhere there. Don't worry that any resort is 'too fancy' for kids - it's not, I promise. We've visited them all and kids are never out of place.
I definitely recommend that you plan your days. Disney does post park calendars 6 months in advance, so check that to help you plan around the parades and shows you want to see. This planning is also essential for making your dining reservations. These should be made as far in advance as possible (currently 90 days in advance, but may soon return to 180 days in advance).
The dining plan is a personal decision, so it's hard to say. It can be a great value if you plan on eating table service meals, and it definitely makes budgeting for your trip much easier. I'd recommend that you visit
www.allearsnet.com and review the menus, decide where you'd like to eat and make your ADR's, and then run the numbers and see if the dining plan makes financial sense for you.
Good luck!