Should I tell my children their father cheated on me?

It depends how old they are. If there under 6 not just yet, maybe and over would be better. But don't leave it to late as they may be hurt by there fathers actions.

Be calm about it, and if they have any questions try and answer them best as you can.

Maybe not if they are young, but if you think they are old enough and mature enough than you should sit them down and talk about it calmly!

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Age 6 strikes me as far, far too young. I don't see how a six-year-old child can be expected to make sense of what is involved.