When you create your account online, you'll be walked through two steps:
Step 1: Create your personal profile
Step 2: Create a child's profile. You can add as many children here as you need.
Once you've completed signup on the web, you'll be taken to the Activity page where you'll see the activity timeline view and your children's avatars along the top left margin. If you only created one child, you'll see a circle with a plus inside along that left margin, and you can click that to create a new child profile.
All child profiles you create on the web are of course accessible via iPhone and Android apps as well (you just can't add new child profiles from the apps yet).
As always, don't hesitate to get in touch with any questions or if you could use a hand with anything. We're here to help.
For instruction on how to remove a child, see our help article on how to hide or delete a child.