Could sit down with the both of them. Tension like that, does noone any good and for the sake of the children, not being able, even just cooly behave professional in group settings, must be so awkward for the children.
You can be sorry that you entered this feeling she'd be accepting, anything after that is their issue, not yours, you couldn't predict the future outcome and thus have enough stess without shouldering blame if they cannot behave civilized.