I have done it with Linux. I just ssh in. sidedoor is good if they are behind NAT. That way you can fix things, install programs, handle updates, etc. There ar

I have done it with Linux. I just ssh in. sidedoor is good if they are behind NAT. That way you can fix things, install programs, handle updates, etc. There are tools to share the screen. But usually it is enough to use X Window to run a program on their computer on your own screen. It is also handy that you can just scp photos, videos etc to their computer.