Vintage Kids Platsic Cup and Saucer set by Campbell, made in Italy

I found this set of 4 plates and 3 cups marked "Campbell, Italy." I was wondering how rare this set is and where can I go to find a value for something like this? Furthermore, when were these produced and how rare/common are they now?

I would love to do more research. There is no damage on any of these pieces, just some yellowing on some more so than on the others.

Thank you!