They do sell plexiglass guards at Babies R Us. That might work to block the on button, but may have to be moved to allow the grandparents access.
Since you mentioned that it is an old "tube TV", is the TV secured so that it can't fall of the table it sits on? This is a big hazard with toddlers. Pulling on that TV could cause it to fall off the table and cause some serious harm to your little one. Don't want to scare you or cause alarm, but I know a family that lost a daughter to a TV fall and just read the news about a child in the hospital recovering from a brain injury due to a TV fall.