I've cruised on a number of Carnival ships, though not personally familiar with the Imagination. Have been on a number of other lines as well, including Princess.
Your kids will likely find many others in their age groups on Carnival. So, they should be happy cruisers.
IMHO Carnival has made great strides in their onboard product. Their food quality stands up against all the other majors very well. Presentation may not be competitive yet.. but it tastes good.
I really enjoy the relaxed and fun atmosphere on Carnival. Go with a relaxed and fun attitude to match, and you'll be sure to have a fabulous cruise.