Note: if you want to point customer's to a self serve article on how to do this on their own, you can share this article with them.
To do this for the customer, from a customer's profile, click Children > Add new child
If the child has their own email and the parent is ok using that email to create this Momence account, just enter their name and that email.
If the child doesn't have their own email or the parent prefers to use a variation of their own email (the parent's email with a +1 affixed to it) to create the child's, this is possible using the the 'Auto-generate child's email' toggle.
Next, click Create > check the emails inbox > and confirm the account.
When it comes to booking for the child the parent will be able to select their child from a dropdown menu on the checkout page.
Note: the option to book for a child will only show if you have this setting turned on in your Momence General Setup settings. (see Settings > General Setup > Misc. Settings)