I didn't think of this before but let's take the child part completely out of the equation.
Let's say you were the one who suddenly and without warning threw up in the theatre. Would you then just sit there and not move? Or would you want to get out of there as quickly as possible?
Now, yes, you might sit there for a minute or two, trying to pull yourself together since you're going to have to get yourself out of there under your own power. But given the choice, wouldn't you rather just get out of that theatre asap? Anyway, I know I would.
So, by the same token, you'd want to get the child out asap. Seriously, would MIL not have gone to the restroom immediately had she been the one to throw up?