I have an eye for little details. Also notice that “Erin” is not really a shorter version of “Ernie”. Which is why I expect it is the name of his kid maybe also along with Tammy. They both appear a lot in those covers above.
I can see the logic of Debby joining the Kid’s voices, but Ernie would be singing back to himself so I think that would make less sense.
I notice that Tammy also appears as an “adult voice” on vol 10 which makes me doubt is she is their kid as it could be a sister… or an older kid. Difficult to know.
And to continue the detective work - notice Vol 10 the copyright is “Rettino/Kerner” publishing. So that tells me Debby’s maiden name is Kerner. So when we see “Debby Kerner Rettino” that is not her middle name, but a two worded surname. How is she credited on the first Release? I bet she still Debby Kerner on there?
Your Vol 10 paperwork is a 1991 edition with 1990 copyrights so “Rettino/Kerner”, but the Vol 9 paperwork is 2006 and instead “Ernie and Debbie Rettino” copyrights. I am guessing these are all the same entity but with a name that is evolving over time.