Hi...the problem of cruising with kids during summer vacation period and holidays is that ships are 100% full of families with kids. On a 2,000 passenger Carnival ship, there may be 700 children! On the popular cruise lines, fares will be at their highest and airfare is high as well.
On ships I've been aboard, I've seen some -- not huge numbers -- of kids. This probably has something to do with the vacation period of the parents. Or, it's much less expensive to cruise then.
Personally, I think travel is one of life's most educational experiences, so I have no problem with kinds taking a week off from school.