There are two separate issues here - your MIL's request, and your husband's response.
I don't think you are being unreasonable about the situation overall, just maybe the arrangements.
Firstly, your MIL is absolutely within her right to have the birthday celebration that she wants. No one should have to arrange their lives around someone else's children, not even grandparents. I'm sorry if that excludes you - it wouldn't fly in my family because being together trumps everything else, but I know that is unusual (and we're Mediterranean where children are just thrown into the mix and left to fend for themselves , which probably wouldn't work for you anyway!).
However, if your MIL is going to be a brat about your husband choosing to stay with his very small baby and high needs child to support his wife, then that is highly unreasonable. She knows the situation, and she's asking him to put her first because it's a 'landmark' birthday (it isn't. 65 is not a landmark anymore than 66 or 42 is).
Your DH needs to be the strong one here. He has to disappoint someone, which is a horrible predicament for him, but ultimately he is responsible for his children, not his mother.
Is he in agreement with you and is only going to keep the peace, or is he desperate to go? If it's the latter, then he needs to find another way to support you i.e hire a babysitter to assist with baby while you take care of your older child, or have you all stay near the meal so he can leave and help you out if needed. If it's the former (he doesn't want to go but feels obliged and didn't want an argument) then he needs to brace himself and have a hard conversation with his mother.
Anyone commenting on how you should suddenly force flexibility on your children out of nowhere is totally bonkers. It doesn't work like that. You can take steps to increase flexibility bit by bit, but I don't know if you have time for that before the meal.
Ultimately, talk with your husband, agree what you both want, and let him solve this problem. It's his mother. He should come up with the solution.