How does the multiple child discount work

When registering siblings, the first child pays the full fee; each additional child is entitled to $20 off their session fees (in correspondence with the number of sessions).

Example: Tom and Bobby each come for one week; this results in either Tom OR Bobby deducting $20 from their fees - but not both. Say Tom and Bobby each come for two weeks; this results in either Tom OR Bobby deducting $40 from their fees - but not both.