I agree with @DianeR. Go without your husband. Tell him you plan to go; invite him and respect his answer.
Then, if he chooses go and is not fun to be around, concentrate on that shining star and your daughters.
I’m sorry to hear how sad your life has gotten. I have a friend whose retired mother and father even live in separate houses miles apart, but go on trips together and are still very married. Life takes us in many directions and sometimes time apart from a spouse is an okay path.
The sadness of your son’s illness and accident will be with you for a long time. I like your instinct to go somewhere joyful, a young child’s birthday party. This might touch into your sadness in some ways, but it’s alright to be sad and feel pain, just as it is alright to be happy and feel wonderful.