I think YABU - it's 90 minutes at a time when two three year olds are likely to be done with present opening and are now either engrossed in their new toys, happy in front of Christmas morning TV or are ready for some quiet time.
Just because it is Christmas Day doesn't mean you have to be together for the whole waking time. I think the fact that you are older parents means that the other runners haven't factored in small children to their Christmas morning run idea, as most other 50yo parents will have older children, but it also shouldn't mean your DH has to miss out all together either.
Perhaps a compromise can be made - a shorter run; he is responsible for any prep the night before; certain things need to be done before he leaves for his run; OP getting a couple of hours either later in the day or on Boxing Day etc - so that everyone gets to be happy on Christmas Day.
Its 90 mins when the lounge floor is covered in wrapping paper, dinner probably needs to start being prepped and there the breakfast stuff that needs washing too as well as getting the kids ready brushing hair and making sure their teeth are cleaned.
Jfc it's Christmas Day, why are all of these things suddenly top priority for those specific 90 mins? If there is one day when normal schedules go out of the window it is Christmas Day.
Two 3 year old will have been up well before 9am so just clear up the wrapping paper as you go and it won't become an arduous 90 minute task.
Every Christmas picture from my childhood features a bulging black bin bag somewhere in the background, part of the present opening included either a parent, me or one of my siblings doing a quick sweep of wrapping paper between rounds of presents.
Dinner prep that is essential can either be done the night before, can be prepped together a bit later, or can be done when the kids are in front of the TV for half an hour.
What elaborate breakfast do you have at Christmas that requires significant clearing up? In my house growing up we'd eat selection boxes at about 6.30/7am during the present opening and then we'd be done by about 8am when mum might make bacon sandwiches or we'd have Christmas crumpets, croissants/pain au chocolate or just our bog standard cereal or toast, none of which required more than a few side plates/bowls being washed up which took 5/10 minutes at most.
As for getting ready ... why does it have to be done at 9am and why is it a massively arduous task? Getting dressed takes minutes and, unless you're going out, with 3 year olds I would have thought that leaving them to get dressed until later would ensure any Christmas outfits were less likely to get ruined or dirty ... since the OPs DH will need to shower and get dressed after his run then he could also be responsible for getting the children showered and dressed at the same time. When DD was a toddler, getting her dressed was practically the last thing we did before leaving the house so that we could have a chance of getting to my parents for dinner and have her Christmas dress still be clean and unstained by chocolate/dirty marks.
One of the best bits of Christmas when I was a kid was staying in our PJs playing with our new toys and/or watching Christmas TV all morning until about 10.30am when we would get dressed in our new Christmas outfits and dad would take us to our paternal grandparents house (where Santa had always left us a few gifts too) so my mum could have a couple of hours peace to sit with a cup of coffee, have a relaxing bath and start the dinner without us getting in the way and then we would be back just in time to entertain my maternal grandparents as they arrived for Christmas dinner.
Hair and tooth brushing again takes a couple of minutes - by 3 my DD could brush her own teeth and hair, I would just give her teeth a quick once over after her effort and style her hair if needed - but tbh on Christmas Day morning tooth brushing was not highest priority (I always made sure DDs teeth were brushed before bed on Christmas Day as a minimum)