Captain Marvel Jr gets the back-up story in Shazam! 18 (May/June 1975).
Maggin and Oskner open the issue with a story centring on Mr. Tawky Tawny. The tiger goes for a camp out in the woods with Billy Batson and Dexter Knox, and comes across a talking frog. The frog's backstory is the usual enchanted prince tale. Although Tawny can hear him talk, no humans can. To check out his story, Knox hooks the frog up to an intelligence meter, which it sends off the scale. Some Russians spies see this, and think that Knox has boosted the frog's intelligence, and steal the creature.
Tawny grabs hold of the helicopter the spies are in, trying to stop it, though it's Captain Marvel that brings it down.
Tawny's act of selfless friendship winds up working the way a kiss usually does, and the frog becomes a prince again, although a frog looking one.
I would happily have skipped this issue, and the lead story, but the Captain Marvel Jr back-up uses Sivana Jr as the villain, so I had to include the book. Bridwell and Schaffenberger execute this tale, in which Sivana Jr creates a coin that subjects anyone who touches it to the villain's mind control.
After a test run at a bank, which doesn't go as planned, Sivana Jr gets Freddy Freeman to hold the coin, and then commands him not to say the name of Captain Marvel, so he will not be able to change into Captain Marvel, Jr. This sort of works, although Freddy realizes all he has to do is say his own hero name, which includes the name of Captain Marvel, and gets around the compulsion that way.
Oh, and I just noticed that this is actually the last new Captain Marvel Jr back-up story. There are two more in issues to come, but they are both reprints.