You are totally unreasonable and extremely unkind.
A child is having her world turned upside down, (regardless of where fault may or may not be laid)
and you intend to add to that misery by blanking one of her parents, in order to satisfy some sort righteous indignation within yourself,
So when this child who's life is being destroyed by adults, a relentless stressful situation beyond her control, goes along to something that fills her with joy, she will be met with a woman who previously has been friendly and kind towards her whole family, wearing a cat bum mouth and judgy pants wedged so far up her arse her eyes are watering.
Your reaction towards this child's mother will hurt the mother, who will then have to deal with your reaction, so it will affect her DD, so you are just adding more stress and pain into a situation which have more than enough without you using it for some sort of personal therapy to boost your sense of self worth.
and by adding your layer of punishment on top of a delicate situation, you are also hurting this man, as he no doubt will need to salvage what he can of family relationships in order to be able to have as much of a parent role.
in such a fragile situation, you along with others like you, might add the straw that breaks this camel's back, at best this woman might decide she can't face your's other immature reaction and take her daughter out of the class, so her daughter doesn't get hurt by other adults(she had enough of that) , or think fuck it ,fresh start and move, thus depriving the dad and DD of regular contact.
If you real gave a monkeys, you would do your best to be really genuinely kind and nice to both parents, and make everyone as comfortable as possible.
Rather than a drama lama for catbum mouth kicks.
Don't forget your child in this you are meant to be a role model, you need to think out your actions, bit more deep thinking a little less bitter knee jerking reactions, as they will be replicated by her, and you are not setting a good example.
There are no winners with your plan, in future when you come up with a plan, make sure everyone's a winner, or it a shit plan, just so you know, clearly no one ever mentioned it to you before, and thinking is clearly not your strong point.