I agree with MM, they shouldnt get it back and i think you should complain again.
This happened to me and my cat Boy in 2007, he was given Norocarp instead of Synulox
Simply because the head nurse didnt check the tablets when i went to collect them and eventhough i phoned to check because they didnt look like Synulox she said they are fine
I do think you should complain and follow it because it could happen to some one else . I did receive a letter to assure me that they make sure every prescription and medication will be checked by 2 memeber of staff , one to be the duty vet from that time on! It didnt do me any good because there were lots of complication after Boy took the wrong tablet,but i am sure it help others in that surgery.
I hope your little one is ok.