I was 9, my brother was 6. We both remember the day we were told vividly, and we were both devastated. We were taking the dog out, and db and I were sat in the car. Dad just leaned into the car and said that sometimes people meet other people that they just can't live without, and he had met someone like this and would be going to live with her.
I can then remember walking with the dog and mum talking about it to us and us saying 'I just want to die if daddy doesn't live with us anymore' (I still feel guilty about saying that to mum.) Dad had left mum for another woman, and although the shock of finding out that day remains with me, life afterwards was fine. While we were young we spent every Sunday with dad, and he would visit us one night in the week, and he did make a real effort to do nice things with us (and bought us sweets 
) which he had never done while living with us.
Both parents, but my mum in particular, behaved in an exemplary way when they split, I cannot praise them enough. They never badmouthed one another, and they always put us first. Growing up wasn't without it's difficulties though. Both mum and dad remarried and had more children. There were stepchildren in the mix too, and me and db often felt like 'the inbetweenies' going to and fro between one 'proper' family' and another. But on reflection, I would always choose my large family with many siblings than the nuclear family we'd have been if mum and dad had stayed together.
Sorry, just realised I've been waffling on for ages 