Whether you are registering for yourself and someone else or you are a parent registering multiple children, the process is the same.
You must create an account, complete the first registration in the usual way.
When you get to the Summary page, click Register another person.
This takes you back to the beginning where you can register another person in the usual way. You can register as many people as you want.
The Summary page will show all the registrations separately and the total amount to be paid.
Continue through the payment process as usual.
For further information, see these articles.