I think it depends on what the shoes looked like. Is a quick clap and rubbing in the grass going to fix the problem? Is it going to take several washings and a toothpick or a butterknife to get all the poop out?
Were the shoes kind of cleaned off, or no attempt was made?
Because to my mind, it makes a big difference. I'm not going to take a butterknife to someone else's kids shoes. But I will try and clap the poop out and rub them in the grass or shoot them with a hose, you know? Because, really, especially if the kid tracked that dog poop TO my house, I don't precisely feel I should make those poopy shoes pristine before sending them back. What were the circumstances with the kid showing up at House B with poopy shoes? Did House B go and pick him up, or did he walk straight there after school? He didn't stop off at House A? Right? Because if he DID, I wouldn't feel ANY obligation to clean off those shoes at ALL. And I'd think it was pretty crappy that someone let their kid track dog crap to my house and then expected me to clean them up. I'd think that's --no pun intended-- pretty crappy and disrespectful.
So like I said, it totally depends on the circumstances, how much poop, how much work it would have taken to get them clean, etc.
And depending on how old the kid was, I'd probably make the kid clean out the poop. I'd at least make him help out. He's the one that stepped in it. And with both shoes, it kind of sounds to me like the kid didn't accidentally step in the dog poop. It sounds like he deliberately stepped in the poop, and that kind of bugs me, too. Unless the kid was wading through it, and it was like, a huge huge pile-- you can't get that much poo on BOTH shoes... and if it was a huge huge pile, I"m sure the kid would have been able to notice it first, and thus have the option of avoiding said dog poop.
So that would also play a part in my decision.