Here is my experience having gone to Disney many times with 3 young kids:
It is much better to have 7 park days and spend less time in the parks vs. 4 park days and trying to cram it all in.
For example, you go to MK in the morning, leave at 1 p.m. and take the rest of the day off.
Or you sleep in, go to Hollywood Studios in the afternoon and stay to close.
This avoids the "theme park death march" I see many families doing. I understand for some it is an expensive, once-in-a-lifetime trip, but if you can avoid doing it your visit will be more of a vacation and less exhausting.
The cost per day drops quite a bit the more days you buy and it's worth it in my opinion.
I'm sure your next question is if it's worth it to buy park hoppers. My 2 cents is sure, they give you flexibility, however, If deciding where to put my money, I would buy more park days.
For example, in a week we might hop twice to see a show we missed, eat at Epcot or view the fireworks. While it certainly is nice, it would just be less of a priority.