Is your younger bitch fed on different food? I had a similar problem but thankfully my boy grew out of it when they were all fed the same brand. Until then I just made sure I was there to pick up as soon as it was done. I've heard the pineapple dies the trick (as suggested by Perry) apparently it makes the poo rather acidic an unappetising.
Cheers
Vicky
It's not how much I love my dogs, it's how much they love me that's important!!!