I’m a bit confused.
Is it that you think your child wouldn’t be safe around your mom, or do you want to shield the child from your mom’s obnoxious behavior when she’s drunk?
Both?
Because that is a boundary you can set and could have set (maybe you have?) at any time, child or no.
“Mom. i will leave when you get obnoxious.”
I, too, recommend Al-Anon . Lots of support there.