This could have been written by my DM. Except we were slightly older when she had her DC (now 4) with my stepdad (there's 25 years between us and I also have another older, sibling).
They were together about 5 years before she fell pregnant (at his request, I may add) and had a great life, meals out, weekends away, lots of boozing, grown up kids who didn't need seeing to and lived their own lives.
Then a very refluxy, miserable, non sleeping child came along, followed by a very clingy, demanding toddler and then endless school runs, parents evening, elderly grandparents (2 have already died) so no babysitting on tap. Oh and no sleep, even to this day.
It's safe to say their marriage is on the rocks and they've already split a few times - they get back and it's good for a while but then it steadily goes downhill again and I don't think it's got much of a course left to run.
Having seen the decline in their relationship is very sad but the reality of it is that at just shy of 50, despite being very active, DM does struggle to cope with the demands of a child at her age and my stepdad (who's only 11 years older than me!) feels a bit cheated, I suppose, that this lively lifestyle they had is no more and they are looking for family friendly pubs, everything planned in advance and holidays in the Carribean are traded for kids clubs in the Med, to try and get a semblance of peace from a naggy child.
I think you need to speak to your DH and tell him how you feel - I know DM is limping through at the moment because she's creeping towards 50 and doesn't fancy a life as a single parent of a young child but they are both miserable and it's horrible seeing her so zapped of life and drained whilst trying to keep up the 'fun' stuff.