Omg. Why do some parents act like this?!?! It's incomprehensible to me, yet my parents have done similar things and much, much worse sadly.
It's upsetting to hear other adults going through hurtful moments with their parents
I'm so so sorry to hear what your mum did. I understand completely your sadness at missing out on your little one's first haircut and the betrayal you must feel at your mum's disrespectful action. And for your father to dismiss your pain by making it only about your mum because she was offended, it's just crazy! He's fuelling your anger and increasing the depth if your hurt.
I agree that it would be a shame for this incident to escalate into a family feud or to break up your relationship completely, especially for your children if they enjoy time with your parents. I also agree that you should take this as an opportunity to set firm boundaries from now on and explain everything in writing. However, considering their attitude from your Dad's email, I just really hope for all involved that they will 'allow' you to explain your reasons and that they will take them in an accept your pain as a result of what she/they did.
They would need to get over their 'being offended as their defence' because this attitude will just make you feel worse, more upset and more angry toward them.
I truly hope that somehow they will understand their mistake and accept responsibility that your mother's action was completely misjudged/out of line/hurtful and just plain wrong. And I hope that she will be 'brave' enough to apologise for causing you this hurt, at least this would go some way in making you feel better, if she doesn't it will be difficult to heal this pain and regain trust, I would imagine.
Please don't feel that your upset isn't valid. This is just how they are trying to make you feel with the follow up email... You were hurt by what your mother did because it was inconsiderate of her, to say the least, this is not just about hair! Of course hair grows back however, your Mum and Dad hopefully will accept that your trust and love for them will not if they don't show you they care about having upset you even if unintentionally! Your relativo shop will remain 'cracker' if they are not 'big enough' to show you they are sorry they upset, It's pretty logical and straight forward...
I hope for you (and them) that they can see they need to validate your feelings in order to move on without scars that will otherwise linger in your heart and resurface ever time you have a small disagreement.
I'm so sorry this happened, it was a really big mistake of judgement your mum (and Dad) made... regardless of how they will respond please find it in yourself to forgive them because if they knew better they would do better so don't hold on to your anger, validate your own feelings. Anyone in your place would feel hurt.
Good luck with resolving this harmoniously x