Well, a family dynamic could be set with 2 parents and 2 kids. You have not only the relationship between the 2 parents to work on/in/with, but the parents to 2 kids and 2 kids with each other. I'm going to start sounding like Kierkegaard here with the relation relating to the related and the relating
When you introduce another child to the family, there is that many more relationships going on--everyone to this new child and each other.
For us it has been a huge transition from 2 kids to three. It isn't because our children are well-behaved or difficult (attention: all children are well behaved and difficult at different stages. Don't get too comfortable
). It is more that logistics are way more complex now. Of course it will get easier as the baby grows older.
Mine are just turned 6 years, 3 years and 5 months.
I think the greatest thing about having a third child is the sense between my kids that they are now a significant mass, they are a 'gang' and they belong to each other. The way my 2 boys look and interact with their baby-sister is beyond beautiful.
So, of course it is difficult; but what else would I do? Read another book, draw more, or hold another beautiful baby?