Eh, it’s mostly semantics. I usually use ‘cheating’ for boyfriend/girlfriend situations or when the level of ‘cheating’ isn’t physical yet, and ‘affair’ for married couples.
In any case, one has to wonder if the boy is even capable of fathering a child yet, as he seems to have all the physical maturity of your average 8-year-old - I’d sure like to see the results of a DNA test on that baby!
Some 12-year-old boys are pubescent, and some aren’t ... and the various adult characteristics can develop at different ages. As it were.
I use “cheating” only when there’s a legitimate expectation of fidelity - that is, in marriage.