You can only book one place, for yourself, each time. Invite friends so they can book their own place, or switch to your child's profile to book for them.
For a friend
Book your own place.
Open the session and tap Invite players, or tap Share to send a link.
Your friend opens it, taps Join and pays.
For your child
Add your child as a child profile if you haven't already. See "How do I add my child's profile to my account?".
Open the Profile tab and tap the switch-profile icon at the top right.
Tap your child's profile and tap Switch to this profile.
Book the activity as normal. You pay with your own payment method.
The club checks your child's age and gender against the activity's requirements. Emails come to you.
Good to know
To book several places at once, or for someone without the app, ask the club. The club can add players to a session.