With so much to do for kids aboard today's jam-packed megaliners, your brood — from toddlers to teens — would have every right to be miffed if they were left behind while you sailed the Seven Seas.

Kids at sea are not an afterthought but, for many lines, an essential part of the cruise picture.

Carnival Cruise Lines, for instance, expects to carry more than 600,000 kids aboard its fleet of 22 ships this year — a six-fold increase from just a bit more than a decade ago.

More than a third of the passengers aboard each Disney Cruise Line sailing are kids under 18. In fact, Disney built its fleet of two 2,700-passenger ships primarily around kids and its family of trademark characters such as Mickey and Minnie Mouse and Goofy. Even lines you might not expect to be especially kid-friendly, such as upscale Celebrity or Crystal Cruises, have initiated special programs and allocated separate spaces for the current generation of millennials.

Among the grandest playpens at sea will be Carnival's two-level 5,500-square-foot kids area aboard its new Carnival Splendor, which debuts in July. This will be the largest play area ever featured aboard a "Fun Ship," according to the line.

As part of a retrofit program, Carnival also is rolling out a new aqua park on select ships featuring a 300-foot-long twister water slide, dual-lane racing slides and water spray apparatus.

Though focused on fun, Carnival has not forgotten that kids don't necessarily care about fine food and, consequently, has created a special menu for its younger passengers, such as jelly chicken nuggets. The line even has a children's turn-down service offering freshly baked chocolate chip cookies at bedtime.

In order for parents to enjoy their cruise vacation and get some private time, Carnival also offers baby-sitting services: $6 per hour for the first child and $4 per hour for each additional child in the same family. (Even on luxury Crystal, baby-sitting fees compare favorably, ranging from $7.50 per hour for one child, to $10 per hour for two children and $12.50 per hour for three children.)

Disney, as you would expect, is no slouch when it comes to entertaining kids (of all ages).

A child-friendly oasis, themed to Capt. Hook's pirate ship, contains a play area with slide and rope bridge, a "captain's closet" filled with costumes, toys and games, and an open seating area for arts, crafts and movie watching. It is designed specifically for kids ages 3 to 7.