I don't think my children have made the massive difference to my happiness that many other people's have. There was a time we thought we couldn't have them, and I think I would have been OK if it never happened. I love them to bits, but both pre and post children have pros and cons.
The little baby bit was awful because I NEED SLEEP, but that's temporary so I don't count it in my analysis.
However, a funny side effect of having children that has been really good for my mental health is the enforced daily routine. I was never that great at being consistent with sleep time, meals, housework, fresh air... Now, if I want to do something every day I just make it a part of the toddler's routine and he reminds me! It's really settled me down to have the daily routine of having small children, as it turns out there's all sorts of stuff I find it easier to do for other people than for myself.