If you can, plan what you are both going to say  beforehand as a couple
Start by saying how much you both love your child/children
Explain that you will no longer be living together
Explain that, while you can no longer live together as husband and wife or life partners, you will always be their parents
With older children, talking generally about love and marriage can help them appreciate that relationships can be complicated and sometimes don’t work out, but that you both want to make things as easy as possible for everyone
Avoid blame and present a united front that focusses entirely on your child/children
Be prepared for possible questions and have solutions for the future arrangements already thought out.