[Dimensio]

As though the only reason adults do the right thing is because we might get punished if we don't.

I can only conclude that Christians who make this argument -- and the related "if I weren't a Christian I'd do horrible things" (I've heard this from a number of Christians) -- are psychotic sociopaths unable to comprehend that they're in a scant minority and that most people -- Christians included -- aren't interested killing, raping or otherwise causing harm to others for personal pleasure.

[DBeers]

As though the only reason adults do the right thing is because we might get punished if we don't.

Actually the argument comes before this... It is that we only know the right thing because of God... If not for God then what is right or wrong is debateable e.g. morally relative and situationally derived... Punishment, Hell or consequence is a result of doing bad...

SO -doing the right thing first and foremost requires knowing the right thing to do...