I travel alone to WDW with my dd's now ages 7 and 9 but we have been going since they were 3 and 5.
I recommend:
Stay onsite - you can't beat the transportation and having tried off site once, its truly worth any extra $$ over the offsite places.
Use a towncar service instead of trying to rent a car or using mears for transportation. For your family of 4 it should run between 75 and 85 r/t + tip for the family. A bit more than mears but you will have someone to meet you at baggage, help you with luggage and take you to your resort. With mears you get your luggage, find the counter, pay, wait in a "cattle call" for a bus/van to your resort and possibly tour 1/2 of the other orlando resorts before you get to your destination. (And that's not even stretching it!).
Most towncar companies will also provide a carseat and grocery stop ($$ invaluable). I use the 30 min grocery stop to buy breakfast, snacks and drinks as well as lunch fixings. This alone more than makes up for any cost differental between mears and towncars. It also helps that we can do breakfast in the rooms and sometimes lunch or dinner, thus avoiding the food court shuffle. We get a cheap styrafoam cooler and re-ice as needed, parmelat milk, juice boxes and soda can be left warm and iced as needed. I can also make sure I have some fruits and veggies as the WDW food can get "junky" My girls love raw carrots, cherry tomato and even cucumbers so a cooler and grocery stop help.
I find that park hopper passes work better than the unlmited passes when traveling with kiddos. If you need a day off, you won't find the "clock ticking" on your UPH and can relax and swim, etc. Since the PHP's don't expire any unused days can be saved for a future trip.
I agree with the poster that suggested to budget for a rental stroller rather than bringing one. Especially if you decide to stay at a resort that offers mainly bus service - you don't want to try and do 3kids, backpack and stroller alone - trust me <g>. All the resorts provide a pack n play so hopefully your little one will be comfy in that.
Plan later mornings for days after late nights like wishes or illuminations. Take it slow and take breaks if/when needed, remember than even at Disney a swim in the pool is great fun for kids.
As for in the parks, get to MK when it opens and have a fav or two of each kids in mind. Try and get at least one fav for each kid and then go from there. Don't try and do or see everything, you will all be frustrated and exhausted. Just relax and enjoy what you can do - sometimes just playing in the fountains or dancing on the lights at Epcot is a memory for my kids.
I like to make a PS for late lunch at a reasonable sit-down restaurant. This is our "big meal" of the day and again helps us avoid the food court shuffle. Places like Cyrstal Palace at MK, Prime Time at MGM or LeCellier at Epcot have a great late lunch and we do this instead of dinner. With breakfast of cereal in the room and sandwiches or even in room pizza for dinner at least I know they get one good meal a day. Take a peek at the soups, salads and appetizers if your kids are getting sick of the typical kids meal stuff. Some of the character meals offer lunch - thus getting a good meal and skipping a long line for the autograph.
Don't rule out Epcot for young kids, its my girls 2nd fav park after MK with MGM and AK as 3rd an 4th.
In closing WDW is the only vacation I am comfortable doing alone with my kids. I feel safe and the logistics are taken care of. If you carry a cellphone, slip a card in a pocket of each of your kids with the cellphone number just in case.
Boy this got long, but if you want to do this vacation its completely "doable" as a single mom. PM me if you want more details or recomendations on hotels or meals etc.
Go for it
TJ