Wow. So far I think I had the biggest sibling discount - 25%. And I still got told that I was too much. So here's what I charged:

FT

$130 / week for 1 kid

$97.50 / week for each additional kid (25% discount)

So, $227.50 per week for 2 kids (total 12.5% discount each)

$$325 per week for 3 kids (total 17% discount each)

PT

$3.50 per hour for 1 kid

$2.63 per hour for each additional kid (25% discount)

So, $6.13 per hour for 2 kids (total 12.5% discount each)

$8.76 per hour for 3 kids (total 17% discount each)

But I guess some people expect $1 per hour per kid is more than enough.