I am trying to recall the name of some educational workbooks and characters that would have been in circulation when I was in 3rd grade (United States around 1974).
These texts were part of the curriculum at the public school I attended and were used to teach Greek Mythology. My recollection was that the books were centered around a couple of fictional kids who "discover" mythology or are led to it by uncovering secrets about it through "teenage sleuth" type adventures. I can't recall for certain, but it seems like there may have been some kind of portal that allowed the kids to actually enter the Greek Mythology realm.
I recall the books had quite a few illustrations as you might expect in a grade school educational books. The books were used as a vehicle to generate interest in mythology.