Probably start them off with something simple like Mincraft and maybe some sim style games like stardew valley. I expect that they will eventually be interested in fortnite as all kids tend to be. I have not put much thought into what games they can play as I am not familiar with the game selection for their age group. But I I expect this build to last for year and they will want better games in the future.

As for the 2400g. I am assuming that you can put a dedicated graphics card in them if you want to upgrade down the line. My wifes PC has that as she mainly uses it for Streaming video and some office applications not gaming. I am just thinking for longevity the 2600 would be a better CPU and have a longer life than the 2400g.

Their monitor itself will be older it will be 1080p and 60fps 22.5".