Sorry it took so long. Here's a brief summary:

• Most studies don't show children to have a significant effect on the parents' happiness, either + or -.
• We need to take into account that children have effects on different types of happiness. On the one hand parents get to enjoy all those wonderful evenings reading to their children, playing with them, seeing them grow, etc. On the other hand they can exhaust us in many different ways (even as little kids, forget about teenagers). And then there's the "side effects" such as having to earn a higher income to support them.
• It seems to depend on the person, and which form of happiness you value more. If you value the type of happiness that children increase more than you value the type of happiness that they reduce, then go for it. Otherwise don't.
• There may be a difference between men and women. One Danish study shows that a first child increases average happiness for women but not for men.
• Having multiple children seems to decrease average overall happiness.
• The teenage years are brutal for happiness, but once the kids move out then happiness goes back up (you get to play with the grandkids and let the parents deal with them).