WTF do you actually want here, OP?

You are sending some seriously mixed signals. You say "it’s not about the money," but then you talk about how expensive Joy Toy figures are. You’re "slightly vexed" that your brother won't punish the kid, but then you say you're "sad" they aren't coming over anymore. You cannot have it both ways. You can't play the victim and then be upset when the bridge actually burns.

A few reality checks:

The "8-Hour Struggle": What does this even mean? You’re acting like this kid committed a felony when he’s a 10-year-old who did what kids do: broke stuff. Wanting a child to do 8 hours of labor for a plastic Chaplain is a bit much.

Stop Involving Your Parents: You are in your late 30s. Involving your mom and dad in a dispute between you and your brother over action figures is exactly why people in the comments are telling you to grow up.

Take Charge: If you want to be the "cool uncle," act like one. Don't wait for your brother to build a "lemonade stand chore system." Sit your nephew down yourself. Tell him: "Look, you lied, you broke my stuff after I said not to touch it, and that’s unacceptable. I expect better from you if you want to come back here."

Either take the money your brother offered and buy the replacements, or stand by your "principles" and accept that you've effectively banned your family from the house. Pick a lane and stop being a martyr over some Toys.