No NO NO NO NO NO NO NO. Do NOT do this! You're not taking one for any team, your ex is controlling you and making things nicer for himself, you're taking one last punch, metaphorically, for him.
DO NOT DO THIS I CANNOT EMPHASISE THIS ENOUGH.
It is NOT your duty to subjugate yourself and your own wants and rights utterly so your ex feels better about yourself. You are NOT helping your children NOT AT ALL. Why would they feel better that both their parents want to dissolve their safety and parental unit? That's just nuts.
This happens regularly when men leave, walk out or otherwise cause a marriage to split, the woman is to have no feelings of any kind and tolerate anything and everything for the comfort of her kids. NO NO NO NO NO. You are also a human being and this is an absolutely basic human right, to tell the truth about what has been done toyou.
Just tell them the simple truth. It is FINE for them to know their dad has chosen to walk away, and in fact a better thing, because by protecting them from this knowledge you are setting them up for more and more disappointment and sadness as he will only get more and more self centred now that he doesn't have to live with their feelings.
He is NOT YOUR FRIEND. He is doing this for himself. He is not doing this for the kids. This is all about him.
If you agree to this shittery you will have to lie about it for many years. How fucking horrible for you, to have to take the blame for something you didn't do, tell twisted versions of the truth to various people and at some point in the future, who knows when or where, you will then have to tell your kids you lied to them.
To protect their father from what? The truth? It's protecting the kids from absolutely nothing. You are NOT a united front. He's fucking off, that's the definition of not being a united front.
You are NOT the one dissolving the marriage, just tell them that. Don't give reasons or excuses, just say your dad has decided to leave, I am really sorry, you will have to ask him why yourself as I can't answer for him.
You are not his punchbag, not his wife anymore do NOT RUN INTERFERENCE FOR HIS SELFISH DECISIONS EVER AGAIN.
And what is more he will be happy to lie to them anyway and make sure they think less of you, the monstering of you will begin soon if it hasn't already. Just tell every single person you know the same thing, calmly and without calling him names. Just tell them the simple truth. He's chosen to leave the marriage. You didn't want it. You are sad about it, but coping.
It is absolutely disgusting that he's fucking up the family unit AND wants you to be his prop and doormat while he does it, and very very standard for these sort of men.
Do not do it. The truth is ALWAYS better than a lie and the truth doesn't need to be every bit of information. Your father is leaving, he no longer wishes to be married. You will have to ask your dad about his reasons, because I can't speak for him.
Oh and things are NOT amicable. What you are saying is he won't kick off too badly if you go along with his lie and make him feel better. That makes him a first class cunt. Trust me, the veneer of amicable won't last no matter what you do to try to appease him.
Oh, and if telling the simple truth ruins his relationship with his kids and makes him the bad guy that's because HE HAS CHOSEN THAT ROUTE. Tell the truth. How fucking dare he try to coerce you into this. What an absolute and total arsehole thing to demand.
Try to get him out of your life as much as possible as fast as possible, just rip the band aid off. And never lie for him again. It's not your job and he has no right at all to ask. Good luck.