Are you implying that the grandparent lived in the 1960s/70s or is the story set at that time (or perhaps as a flashback). When I think about my grandparents I don't necessarily think of them using outdated lingo. For a picture book, with a generally low word count, I wonder if such phrases would be necessary to evoke the "voice" of the grandparent, so to speak. When you have limited words, you want every word to count. Again, it's hard to tell without reading the actual book or knowing what you're going for. I do agree with Debbie though, talk to people who are the same age as the grandparent(s) your writing.
Also keep in mind that popular lingo will be influenced by a lot of things; geography, socioeconomics, heritage, personality.. even if the grandparent in question is just a minor character it might be helpful to think about where and how they grew up in order to really capture their voice. Again, though -- for a picture book you're not going to be able to include all those details in your story. They're more for your personal reference in order to get a feel of the character.