You don't have to make a fuss. You simply hand the cashier the games and say "I'm sorry, I didn't put these in my cart." I'm guessing you get a little anxious, because that would explain not ending this at the cash register. It's also ok to say "I'm sorry, cancel my order, I have something I need to take care of" and leave with your son. If your son manipulated this situation knowing you would be too anxious to stop him, then take some time to practice dealing with it. Worst case scenario is what, a bunch of strangers think you're a harridan? To heck with them. Most people will be wise enough to understand you're handling something and they're not aware of all the particulars. Many people will be silently applauding or thanking you for dealing with this before the kid is an adult. You don't mention how old the kid is. I am hoping you returned the games and did not allow him to have them. If you did, he should be paying you back.

Since you ended up buying them just make sure he never gets to play them. Let his siblings, friends, cousins whoever play them but only if he doesn't. He'll feel like shit and chances are he won't pull that again.

Take it back. I don't know about your country but in mine you have 15 days to return any item you purchased, except food. And ground his ass because money doesn't grow on trees; if he wants 350£ worth of games he can go earn those 350£.

Return them! Preferably while dragging him along. You can still salvage this! Unless they don't take returns... For your sake I hope they do.

I have worked the till at a few different stores during my lifetime. You can generally tell the employee at the register that you have decided against buying something because it happens all the time and most (if not all) stores are prepare to deal with it. There's no need for a big song and dance. The only time it is frustrating for the store employees is when the item in question is a perishable good. Then the employees have no way of knowing how long the item was removed from it's temperature controlled space and they need to determine if the item is still safe to sell. That can be annoying. But video games? The employee would have just shoved them under his or her till and put them back on the shelf when he or she had the chance (or called someone from the appropriate department to do it). Embarassed or not, there was no good reason to actually buy the games.

Honestly I don't believe OP has a right to be upset AFTERWARDS. She should have just calmly handed ALL the games to the cashier & stated simply "I am not buying these, my kid needs to learn he can't just have what he wants when he wants"...purchase what IS necessary & deal with her kid after she gets home.

resell them or make him pay for each and every one

you have to be a lot firmer than that if you're going to raise your son. you're a parent, not a doormat

Make him pay you back. The full £350

So you paid a shit ton of money for games that your son disobediently snuck into the cart, so you wouldn't cause a scene in public? I don't think you're ready to be a parent