I started out with a sibling discount. I gave 10% off the fee for the second and 15% off the third, etc. Wouldn't you know it, my first clients were a set of 3 kids? I realized how much income I was losing out on, and these particular siblings were at each other ALL.... THE.... TIME!! Grrrrr.
Then I changed it to 10% off the fee for the second or third, etc kid.
But I no longer offer it. I know some do; but I agree that I am not giving that kid 10% less food, or doing 10% less activities with them!

PS- 50% off??? Holy cow, they scored on that deal!! Lesson learned, huh?