If possible, buy them an old iPhone 7/8/10/11. These are fairly cheap at this point and hard to brick or infect.
For PC, just install ChromeOS Flex! It is super barebones and locked down, so even if they download stuff, it is hard to install anything and File Explorer does not give direct access to OS files. You can Turn On linux dev env and install some interesting applications using flatpak, which are adequate and still has limited access to everything unless they are savvy enough to go tinkering around with apt tools.
For PC, just install ChromeOS Flex! It is super barebones and locked down, so even if they download stuff, it is hard to install anything and File Explorer does not give direct access to OS files. You can Turn On linux dev env and install some interesting applications using flatpak, which are adequate and still has limited access to everything unless they are savvy enough to go tinkering around with apt tools.