Get the Raspberry Pi 5 with 8GB (or more if you care). Get the RasPiKey [1] for faster storage (compared to SD cards), without needing to get an SSD. Just use the Raspberry Pi OS.

Re: software setup, curriculum etc.. there are so many resources that target the Raspberry Pi that you can choose something that the child finds interesting.

You can do a desktop computer build if/when they outgrow the Raspberry Pi.

[1] https://www.uugear.com/product/raspikey-plug-and-play-emmc-m...