Keep in mind that while having cool old receivers is fun, they are old. They have old capacitors, perhaps freyed power cords and are filled with dust.
I would first inspect the recievers for any defects, especially with the power cord and switch for possible fire hazards.
Next, I would plug the receiver into an outlet strip that has a switch on it. That way, when your done listening to the vintage receiver, you can switch it safely out of the 120 volt outlet. I would never leave these old receivers plugged in as one day you may come home to smoldering ruins.
I'm glad the kids enjoy them. Now get them a turntable. aa
W