As to whether or not to buy hoppers since you are only going for 2 days.......well that depends on a few things. Are you planning on going back to WDW in the next few years? Is money a big factor right now?
If you are planning on going back in the forseeable future and you can swing the cost of the hoppers, then I would get them. The unused days NEVER expire and can be used on a later trip. Plus the gate price for one day is more expensive than the price per day on a hopper. So you would spend more now but save in the future. Also if you buy the hoppers you can take advantange of the advise of the previous posters. One day tickets are for that park only and you cannot hop to another. So if you go to AK as one of your days - it closes much earlier than the other parks. If you have a one day ticket, your day is over. But if you have a hopper, you can spend the evening in another park and get to experience more.
As far as which parks to hit in 2 days. With your kids ages, I would definitely agree that MK is a MUST. But I would have to debate between AK and MGM for the other day. While I love Epcot, it does not have as much to do for kids as young as yours. So since you have limited time, I would pick between AK and MGM. Both have things in their favor - just depends on what you like. I know my kids love Playhouse Disney, Muppets, Little Mermain, Beaty & Beast, & Indy at MGM. But since they both really like animals they also really enjoy just walking around and looking at all the different animals at AK - not to mention the Safari, the Petting area at Planet Watch, The Lion King show and playing in the Bone Yard. So just depends on what they like best.
You can schedule a character meal at any of the parks. Depends on which characters they like best as to which one to try. My kids really like the Pooh characters so Crystal Palace in MK is one of our favorites. We usually schedule the earlier lunch setting as we are not big breakfast eaters and the cost of breakfast is a waste on us. This way we get to the park early and hit the big things in the early morning and then have a nice early break for lunch. We are then out when most everyone else is sitting down for lunch. Has worked out well for us in the past.
Good luck deciding.