I don't have kids and it is highly hypocritical of me to say so but "Leo Tolstoy wrote War and Peace while living with his wife and 13 children."
Was War and Peace a side project or the main project?
If it wasn't done as a side project then it isn't really relevant.
War and Peace was the side project. Tolstoy's main project consisted of managing the land and peasents on his country estate.
Tolstoy was one of the richest men in 19th century Russia. His situation isn't the least bit comparable to a middle class nuclear family of our time, where parents actually do the work of raising children and hold down jobs as well.
For the first few months, having kids meant zero time for side-projects, but these days it just makes me much more focused in the free time that I do have.