Being totally honest here. It affected my childhood in a big way - and had a worse impact on my brother, who missed my dad dreadfully, and clashed with my stepmum and stepdad. I was 9, he was 6 when dad left (for OW).
I was a child in the 70's, so blended families weren't so common then. It affected my brother and I in so many ways it would take me an age to describe it all.
Both parents quickly went on have more dc with their new dp's. Db and I always felt 'caught in the middle' of two 'complete' families - moving back and forth between the two. My half siblings on both sides had much smoother, more advantaged childhoods than DB and I did. We were constantly sidelined by both step parents. My stepfather told me and brother that if we failed our A levels and didn't get to University, "we were out". Of course no such threat was given to his own daughters, who have been lavished with both money and support.
Sorry it sounds bitter. Despite what I've written above, my childhood was generally happy, and I adore my half and step siblings. I have a huge extended family now - and IMO that's a good thing. I certainly have led a more interesting life than if my family had remained the usual nuclear family.
But problems of divorced parents have continued into my adulthood. My stepmum insisted on coming to my graduation day, despite me only being able to get 2 tickets - one for mum, one for dad - and hung around awkwardly all day. Arranging my wedding was a fecking nightmare.
But speaking as an adult, I would never have expected my parents to stay in an unhappy relationship just for my sake - something tells me that would have been worse. I still remember them bickering all the time, and feeling that they didn't like each other that much.