Let's see if I can answer some of your questions.
Yes there are occasionally lines but they go fast and usually are for stuff that you choose to do such as Princess Signings or Professional Pictures or Pictures with the Characters.
Dinner lines are just because folks get to the restaurant before it opens (like 8:15 for the 8:30 seating) and have to wait till they open up again at 8:30.
We had the 8:30 seating on a full ship and often got to the show within 5 minutes of the start time and never had a problem getting a seat. I'm sure it was the same for the other seating.
FYI, the major shows are performed twice, at 6:00 and at 8:30 so everyone gets a chance to see the show. You go to the one opposite your dinner seating.
We didn't go to the family entertainment in Sessions or Diversions or Beat Street so I can't tell you how crowded they are. They're on the agenda for next cruise.
The only way your grandson WON'T see lots of Disney Characters is if he wears a blindfold the whole trip!

The Characters have scheduled appearances and then there are times that they just roam around the ship. They also make special appearances in the Club and Lab. There's a number you can call from your stateroom to find out about Character schedules and their schedule is also shown on a monitor in the Atrium. They are also part of several of the shows in the Theater.
Sometimes there are more people around the Characters than other times but I think NOT seeing Characters is the LAST thing you need to worry about.
Yes there's usually some kind of line to get on and off the ship at the ports but that's because they swipe everyone's Key to the World (KTTW) card as you leave and board the ship so they always KNOW who is on and off the ship. They also scan everything you bring back on board the ship in a scanner like at the airport for security. These lines move pretty quickly and I would rather wait a little for the security it provides.
There's a line in the Terminal because folks get there early and then have to wait till they start boarding. But there are also characters in the Terminal and a really neat model of the ship as well as TVs showing Disney cartoons to help you pass the time.
The lines you will encounter are NO WAY like the long lines you find as WDW if that's what you're worried about.