Powerful CPUs are going to generate heat and likely require active cooling. Other components (ie. Chipsets, RAM, HDDs and SSDs) also may generate enough heat together to need case fans. Typically, more computing power means more heat will be generated.

I have an old Dell Wyse 5010 thin client with Debian 11 installed. It is fanless, but also has an extremely low power x86 CPU, is limited to 4 GB RAM, and USB 2.0. Some newer Dell thin clients, like the Wyse 5060, also can take RAM to 8 GB with 64 GB SSD.

Also, as a general rule, enclosures (cases) with greater free air volumes can use larger and lower-speed (ie. quieter) fans to evacuate heat.

So, your best options are…

1. low power ARM SBCs,
2. low PCs or thin clients with x86 CPUs that have very low thermal design power (TDP), and/or
3. locate the computer(s) in a part of the home where your nephew will be near them only when needed and only in short bursts. Connect to them through network cabling so he can work on them remotely,