This has long been a part of the Muppet mythos...when TMTM came out, there were a whole bunch of press interviews showcasing the wedding scene with Piggy saying that they were now married since it was a real minister playing the part (and it was a real minister playing the "real minister") and Kermit saying it was a scene for a movie. They've actually riffed on this since TMTM promotion long passed. Though the kids in MCC aren't really their children - they've been noted in outside materials as being actors brought in to play their kids (much like Robin, Kermit's nephew, played Kermit/Bob's son)...one MCC book at the time even had a back cover listing the cast list of the movie...ie Bob Cratchet: Kermit, Fozziwig: Fozzie Bear, with the "real names" of the actors playing the Cratchet children included!