No brainer then, El Capitan is essentially Yosemite (Fixed).
Do you have a fairly large memory stick? You could create an install USB from home and then take it to your grandparent's place
Unless I missed it you have to have a Mac to make the USB stick, which is my problem - I don't use a Mac anymore myself. I'll just bring their computer home one weekend. It's the 21.5" so easy to transport.