Wow.
From the responses, I hope we're all able to say we NEVER left our kids asleep in the house while we went out to get something in the car, walked out to the mailbox, stood on the front sidewalk to talk to a neighbor for a few minutes......
I hope we all can say we NEVER took any medication for allergies, broken bones, recovering from surgery, etc, or had a glass (or two) of wine that may have impaired us in some form.
That said, I get that your mom has had problems and I get that your husband may not trust her. Exactly how can she regain his trust and how many years does he believe it should take? Have you come out and asked him that question? If he says he's NEVER going to trust her, then you have a real problem.
You tried to compromise by letting the older two kids be with her and other family members. I don't know how mature your oldest is, but at 6, my kids both knew how to use a phone. I understand that "hoping" your mom would be on her best behavior is different than having certainty, but how will you ever have certainty if she is never given a chance?
Just saying.
I don't think you should refuse to go all together.
Leave all 4 kids home with your husband, let him figure it out, have a great time at your reunion and spend some time with your mom.
Your older kids will be disappointed. Your mom will be disappointed. The cousins will be disappointed. Don't carry the weight of all that. Your husband has drawn a line in the sand. He can't stop YOU from going, having fun, and having a relationship with your family.
He has trust issues and he may well have a right to them. So...HE can take care of the kids while you're gone.
Problem solved.
For now....because when you get back, you will really have to delve into what it's going to take for your mom to ever gain trust back.
I see that many moms say she must EARN it, and two years of sobriety isn't enough.
How can she EARN it and how many years is good enough?
That's between you and your husband.
Go yourself! Have a great time. Get some perspective. Don't let this ruin your reunion. This issue will still be here waiting to be resolved after you've come back from something that's important to you.
Best wishes!