Every parent knows that children need somewhere to play and run off energy. Most apartment buildings in developed countries have their own jungle gym of some sort for the young children living there or are very close to a park with a jungle gym for them to play on. Young children need to be able to move and climb and swing and play in order to develop full use of their limbs and coordination. If you plan to have young children on your ship, even for a little time, you will need to have a place for children to crawl or walk around to various toys and structures with other children near their own age while their parents watch nearby or a babysitter does.
This may also be a good time to mention that many of the children that work on ship will likely have parents that work at least a few hours a day. Those children will need somewhere to go and you should probably have some sort of daycare plan set up or a network of parents that have it set up so someone is always awake enough to watch whatever children are unsupervised while their parents work. It may be easiest to set up a room where the younger children are watched by the older children and the older children have some incentive, such as money or computer access, to not wander off and leave the children alone.