Hi, Aimee! If you purchase a 2 day Magic My Way Ticket, you can pick any 2 parks to visit. With the base ticket, you cannot hop parks in the same day, but you can choose a different park each day. For example, you can choose Epcot for your first day and Hollywood Studios for your second day. These days do not have to be consecutive, just in case you want to take a break between your park days.
You will not need a ticket for your 2 year old; children under 3 do not require a ticket.
As far as adding a third day, this would be up to you and your vacation budget. Check
HERE for ticket prices. If you just do the 2 days, with your kids' ages, I would suggest you choose Magic Kingdom and Hollywood Studios.
If you decide not to add a third day of park tickets, try enjoying a day at your resort. All the resorts on property, and many off property hotels offer great pools and activities. We always try to plan one day just to relax and have fun at our resort.
Have fun planning your vacation!