and got all passively pleasantly aggressive on his ass with
"Please could you let me know the PO rules about a 10 year old collecting family parcels" (smile sweetly)
"Oh it was your manager was it, could I please have a word" (she'd gone out within the 5 minutes it had taken DS to relay the story so couldn't speak to me)
"Oh dear maybe she could call me if there's a problem here. DS is very upset now because as he left he heard you say 'How disgusting' and of course to him you're someone in uniform and he is very law-abiding"
"Oh it wasn't you was it? Oh it was someone in the queue. Yes I will take your heartfelt apology to him, yes I understand that you're sorry the little man is so upset. Thank you for your time" whistle sweetly, smile gently, walk away.
AIBU to think he won't be commenting on someone's parenting ever again 
(DS(3) is almost 10. He has little jobs to do, one of which is to collect parcels from the PO when we know what they are. The PO is two small quiet blocks away (3 min walk). He relishes the responsibility as it makes him feel like his older brothers.)