After some research online I decided to skip the hair spray trick and spring for some 'fixative' from the craft store. I was still nervous, so before doing the piece I would hang above the crib I did a smaller test canvas. I painted a typical piece of stretched canvas with 2 coats of chalkboard paint, sketched a quick graphic on it, and tried out the fixative. By the time it had enough coats that the chalk wouldn't smear, this is what it looked like.
I was pretty disappointed. I didn't get a before picture, but I think it's pretty obvious that's really faded. I ended up setting the project aside for a few weeks. Yesterday I decided I might as well trace the picture and try the hair spray- it's not like I had anything to lose. Here's the picture with the chalk before the hair spray.
And here it is after 2 coats of hairspray, smear proof:
It's a little shiny but overall much better. I'm happy with this, and now I can proceed with the bigger one I want to do for above the baby's crib. The moral of the story is- try the thing you have under your bathroom sink before wasting 7 bucks at the craft store.
Here's the fixative spray I happened to buy (there are a lot of choices so some others might work better) and here's the hairspray I happened to have (that I bought like 4 years ago- we don't go through hair spray very quickly around here.)