We went 2 years ago, when my children were 6 (DS) and 3 (DD). We did buy the Park Hoppers as we wanted to see all the parks (except for Epcot), and also have the option to go back for fireworks at any park we chose. We did not have water park tickets.
We were surprised at how well the kids did. We were up early every day, had breakfast in our room, then went to Early Entry when it was at MK, MGM, and AK. We stayed until mid-afternoon, came back and either went swimming or took an hour or so nap. Then, went back to the parks in the evening or Downtown Disney.
We did rent the stroller, so that was a huge help with the kids. I do recall my daughter fell asleep twice in the afternoon on the bus on the way back to the hotel. One thing my husband and I both said was that next time we would not get tickets for every day we were there, as it was not necessary -- just nice to have a day or two to hang out at the hotel (we were there a week). The kids just loved swimming in the pool, and were content to do that just as much as going to the park. So, this time (in little more than a week!), we will have 2 days to just spend at the hotel and relax, go to Downtown Disney, just take it easy.
I think a lot depends on how your kids are in general. Mine are both easy going and my son never seems to be tired; my daughter just needs a nap and then she's ready to get going again. So, we were able to pack a lot in on the days we did go to the parks.