Probably the easiest way to run it is with Vinegar, a Wine wrapper for Roblox. You can just install it from Flathub (or Discover on Steam Deck in Desktop Mode), make a configuration file in terminal to set it to Vulkan and then you’re good to go after that:
mkdir -p ~/.var/app/io.github.vinegarhq.Vinegar/config/vinegar/ && { echo "[player]"; echo "dxvk = false"; echo "renderer = \"Vulkan\""; } >> ~/.var/app/io.github.vinegarhq.Vinegar/config/vinegar/config.toml
I know there’s a launcher called Grapejuice that’s very popular for running the game as well, but I don’t know if there are any additional steps needed to get it to work.
Make sure your kid’s safe on there. I know that sounds an awful lot like pearl clutching, but besides the general dangers of children being online Roblox is exceptionally exploitative of anyone creating content on the site(where have we all heard that before?)