If you already have an account, click on Settings on the left panel. Multiple setting options will appear and you can head over to the Family Members/Children to add a kid.
When you set up your account, you can click Add Child Account to add a kid to your family. Fill in their name and create a username and password (e-mail is optional).
After they've been added, you can manage their profile from the Family Members setting. You can enter additional information, like their gender, but they're optional.
Add their weekly budget and auto allowance to set up their points system and start making Strides!