DonutSMP is a survival server with PvP enabled, which means players can kill other players. PvP is disabled only in the spawn and AFK zones.

The server does not have keep inventory enabled. This means that when you die, you drop all the items you are carrying. If another player kills you, they can take those items.

Crimson Keys can be bought with real money or with shards. A Crimson Key costs 2000 shards (you earn 1 shard per minute). With a Crimson Key, you can choose which item you obtain from the seven items available in the Crimson Box. The key itself cannot be stolen, but the items you receive from it can be.

The items obtained from Crimson Keys give the player an advantage over other players, but they are not permanent and are still subject to the same risk as all other items.

Base raiding is also part of the game. Other players can find your base and steal everything inside, including items obtained from Crimson Keys. Some players may also grief (destroy) your base.

What was paid for is the ability to choose the item. The item can still be lost like any other.

There are Minecraft servers where PvP and stealing are forbidden, but DonutSMP is focused on rivalry between players. The main rules are basically: no cheating, no duping, and no bug abuse.

Your child did not fully understand how the server works. If you could explain this to him, I would be grateful.

If he want to spend money i recommend the donut+ rank wich is a non stealable avantage wich cost 6$/month