My children were 4 and 3 when we estranged from my ex MIL, over 40 years ago.
We just told them we weren't happy with the arguments, and we wouldn't be seeing her.
They didn't query it, and were quite happy. Children that age live in the "here and now" and forget about what they are no longer doing, and who they are not seeing.
They had loving grandparents (my parents), various relatives, and didn't lack a thing.
As 30+ adults, they did see her. Once. They came out after an hour and said they wouldn't be going back.
Last year they were contacted on social media, to say she'd died, giving funeral details, and told she'd left them a letter.
They didn't go, and didn't pick up the letter.
Neither myself or my ex ever regretted cutting her off. We did explain more to the children when they were adults,but we didn't go into very much detail really. No point in rehashing the past.
Life's too short to put up with endless chaos and conflict from others.