Settings->Add Child->Child Profile->Save
You can add another child to your account by going to the main app settings (indicated by the symbol with three dots on the bottom navigation. From Settings, you will find an + Add Child link under the Children subsection. Tap the link to fill out the child profile for the additional child and tap save to complete adding the new child profile to your account. You can add as many children as you have to your account via the method above.