I cannot imagine this happening.

Little girl once had a diaper so foul on a plane that *I* needed to change my pants (she was on my lap). My sense of smell isn't the sharpest, but for her comfort--and my desire not to deal with diaper rash--I'd change a bad diaper as soon as feasible. If someone is letting their kid sit in their own stink like that, I can't imagine what's going on at home.

Likely I'd not say anything and let the parent deal with the unpleasant consequences later. Airplane bathrooms are small, but you can change a diaper in them.