3 day base. Or depending on what your departure day is and your budget - why not 4 day base? This would allow some time the evening you arrive and the morning you depart. The price "per day" goes down substantially when you start adding more ticket days - but not sure how much of a savings per day to go from 2 to 4. I know for us to go from 5 to 6 or 6 to 7 is actually very minimal.
We have done hopper and just base - I lean towards an extra day over hopper for several factors:
1) An additional day allows for more of the unexpected - someone isn't feeling well, a ride is closed, weather is bad, etc.
2) Your kids are young so you can still come and go to the same park that day taking breaks as needed (And with the
MVMCP you in essence could "hop" that day anyways) It is exhausting so spreading out over 2, 3, or 4 days makes it more manageable.
3) On your arrival day you could do some evening activities in a park and then not feel like you have to try and get back another evening and actually try to get the little ones to bed at a fairly decent time hopefully making the trip more enjoyable for all (on the non MVMCP day)
4) With FP+ and park hopping - we have found it best to rope drop and then FP+ later in the day at the second park. With little kids, there is no guarantee that this will be the best approach because as the day goes you may need to wake a napping toddler to make a FP+ (not a good idea!). Scheduling the fast passes earlier in the day when you know your 1 and 5 year old will be at their best - is probably a good idea. Then you can adjust the rest of the day on how they are doing. However, I'm sure others have different strategies. Only you will know what will be best for your kids.
5) More park ticket days allow for more flexibility to enjoy the other aspects of Disney such as the pools, downtown disney, etc. With just two park days there is the tendency to feel like you need to "march" onward so you don't miss anything even when all the 5 year old wants to do is swim in the pool. (And yes, my DS has been able to swim in the pool in December.) And speaking of December weather, usually there is only 1 or 2 days that are pool worthy since the weather can be all over the board - sweatshirts to shorts within the same trip. So, this would allow you to leave the park if wanted to enjoy the pool on the warmest day.
Lastly - whatever you decide - enjoy! My biggest tip from having taken a similar trip when my kids were around the same age is to really let them set the pace and don't worry about what you miss but instead enjoy making memories around those things you do get to do. We missed quite a bit because my boys just wanted to hang around Tom Sawyer Island. Those memories I will have forever and I truly can't even tell you what we didn't get to that trip!
