I would find out what is offered for the under 3 (don't think there is any nursery/maybe babysitting?), plus the 3 yr olds must be potty trained to go into kids club, I think. RCI does a really good job in the kids club for the 5-11 age group (never used for under 5). My kids loved RCI, even more than
DCL. They are a more structured IMHO. The 8 yr olds might be old enough for the rock climbing. RCI has a program that kids can get their food ASAP in main dining, then the youth counselors come to the dining room to take them to the kids club--pretty nice. Buffet is open for dinner, so you can also feed kids early and then check them into kids club @ 7 and dine later, if you want. RCI kids club is open 9-12, 2-5, 7-10pm on sea days (of something like that), all day on port days. You can pay extra aftger 10.
We also really like Princess, as do our teens. Never used their kids club. There should be a decent number of kids on an Alaska cruise, but there will likely be an older crowd. P will have kids clubs/activities. Look on cruisecitic.com for Princess info--many say Princess is #1 in Alaska. I think MDR food is a bit better in Princess, rooms just a bit nicer.
What you will miss not on a DCL: different style kids club, more high tech in kids club, dedicated movie theater (RCI plays very few movies, Princess has 1 per night (pg/pg13), plus plays repeat of those in big performance theater on sea days), fun rotational dining, broadway quality pm entertainment, larger rooms with split baths with better layout for families.
Also, in Alaska, you don't have to do a tour. My parents stayed in ANC for a few days and had a great time going to the excellent zoo, cultural center, etc. With kids, I might skip the land portion and just DIY for a few days to be able to go at their pace.