Since it was a different girl who did the cut for free, who could have possibly gotten paid for another cut I would have tipped. You aren't obligated to tip anyone, but as a hairstylist, it would have been nice since you didn't request a length change the first time around with the original stylist. I get if a little girl has a weird length on one side that isn't perfect that you don't notice when you get home, but if it is simply because you didn't like the length and didn't have it fixed the first time around, then yeah, you should have tipped her.
PS>>>> In SOME salons, the original stylist will lose her commission and it will be given to the stylist who fixed it, though the original stylist can still keep her tip. This is how it worked at my last salon. :)