IMHO, the transaction was over after you left the store. You brought it to their attention BEFORE leaving, and I assume would have paid at THAT point, if it was an issue. It wasn't, they "let" you go, and you left. End of transaction.
Personally, I would have simply paid for the mug. Sometimes stuff happens with kids. The $10 may be worth the "life ain't fair. You break it you buy it" lesson.
If you son never touched the item would the mug have fell ?
Your here asking so what do you hope to gain ? You already know the answer. It seems you want some validation or something. Your son should have been more careful when replacing said item . Obviously it was placed there previously without issue ? What changed from that time to this one. By that logic I saw the drunk driver leave the bar and put myself in a position where he could hit me . Lol