WhoGivesAFlying - that exact same thing happened to us 'this time X years ago we conceived dss'! I was absolutely livid as DP had just replied saying 'time flies!' like that was a perfectly normal thing for her to say to him. When I asked him about it he shrugged and said 'what am I meant to do about it?' I found it so inappropriate because she was basically just saying 'Oh hey, remember that time we had sex?'. I'm pretty sure conception is not a celebratory day 8 years down the line.
OP, have you spoken to your OH about it? My OH has never sent a text asking for the messages to stop (except for the extreme ones such as underwear pictures) because he thinks it will start an unnecessary argument, but he just flatly ignores the ones that aren't about the children and even if she texts him about something that is appropriate, if it's 10.30 at night he wouldn't reply until the next day (unless it was an emergency).
Having the ex in your life constantly is definitely hard, my OH himself admits he has no idea what its like for me and he's sure he wouldn't be able to handle it if the shoe was on the other foot. But I just ignore her messages as much as I can, take a deep breath and pick my battles carefully. I think you should say something in passing like 'I think these late night texts are getting a bit intrusive, how do you feel about maybe asking her to respect our evenings unless it's urgent?' I would never expect my OH's ex not to text him in the evenings to let him know if one of the kids is sick, or if we have contact the next day and she's forgotten to tell him something - but a football match in two weeks time doesn't need to be discussed at 10pm.
But then having said that, if I was in her position and I remembered late one night about something that was coming up in a few weeks time I'd probably just text it over while I had it in my head without thinking! It's a total minefield, and having that third wheel in the relationship that you will always have to take into account is tough. I think that to a certain extent it all depends on your OH really, how he responds to how you feel, whether he can see things from your perspective and can adapt to help you all get through it and ultimately, if you want to be with him enough to overcome it. Being with a man with kids and an ever present ex can be a very hard life if you're not 300% into it, warts and all! If you want to stay, communication about your feelings is a must or you'll end up resenting him and possibly the kids too - so after all my waffling I really do think you should just talk to your OH.