Welcome to the forum.
You'll need to update the NOW TV app on your box. Press the Home button on your remote to get to the homescreen, then go to Settings >System > System Update and see if an update is available. If not, leave it a while and try again later. You should notice the NOW TV app is now a yellow and pink logo.
When you open up the updated app you'll see the kids section under Movies, Entertainment and Sport on the menu bar on the left of the screen.
@Anonymous User Just to be clear, what device(s) do you have? 🙂