Well, first of all, and quickly, talk to her dad and grandparents and let them know how you decide to handle this. Tell them they need to respect your choice about whether or not to tell her because it will confuse and upset her if you don't tell her but they do. If you want J. to reinforce general safety and not talk about this attempt, they need to do the same and not bring up details.
On the other hand -- If the school is sending home a note about this, I would bet that most kids, certainly in the upper elementary grades, all know about it and are talking about it a lot in school. Rumors probably are spreading. She may already have heard about it and may know more than you realize -- and being kids, the students may have details wrong or may be blowing up the story to something even worse than actually happened. I would start talking to her in general terms about remembering safety, not getting into cars or even near cars, etc. and see if she seems uncomfortable or seems to be reluctant to say something to you -- she may want to talk about this if she's heard about it at school, and you need to be open to the idea of talking to her about it. An "any questions? You know you can talk to M. about anything, any time" approach would be good. See if she brings it up.
There are good videos called "The Safe Side" which explains safety around strangers. Available at lots of public libraries or online. Not scary, and they do not show kids ever fighting someone off.