We've done many trips both ways. In the old days, hopping was included. At another point we had AP's- which include hopping. We've also done many trips without it.
The age of the kids in our party didn't make much difference, especially as we used strollers. Any out of shape adults usually tire before a kid in a stroller. Ditto for anyone who skimps on their footwear! the kids sit in the stroller much of the day. If they really get tired, they fall asleep in it. At least most kids.
Hopping is mostly only an advantage if you are staying at one of the Epcot resorts and plan to eat dinner in the WS- OR- you are staying on the monorail line and plan to go to MK after the other 3 parks close.
Flat out hopping between parks takes time, especially hopping to/from MK. HS to Epcot is fairly short, but that's about it.
I should add, we are kind of commando-types. At the very least, we try to use all the peripheral hours. If you are more the 9am-5pm type, then I wouldn't pay to add hopping.
I agree with posters who say kids of all ages tend to LOVE pool time. The shuttle boats and monorail are also big hits. Some trips, family members have taken afternoon breaks (but mostly to swim), but not always.