N.,
I'm going to respond first, then read all the advice you get after! I have this same problem and I'm freaking out. Mu hair is fine, but use to be very thick. It's been going on with me for about 15 years and it has stablized, but just got thin. Not anything noticable to anybody but me. I have tried many things, and this list is just to help you not to bother with some of this...Hair and nail vitamins, Nioxin, Big, Sexy, Hair. While these things did help slightly, it wasn't much. Now I did use Rogain for a while and that does work, but if you stop loosing it, you will re-loose the hair you gained. Rogain can also make you slightly dizzy which is why I don't use it anymore. Omega oils are helpful in stabilizing and thickening your hair. It's also good for so many other things.
Also, do you use Pantene? I love it, but it causes my hair to thin when I use it every day. Other people I've talked to have the same problem. If you use it, you can rotate it with other products. I have also heard that a DHT blocker is very good - I haven't tried it yet.
Okay, having said all of that, it is possible that this is just a reaction to giving birth. This does happen and then the hair grows back. Was the second child a different gender? That can cause temporary hair lose. Are you stressed toooooooo much? That hinders hair growth too.
Alright - that's all the useless information that I have on this subject. I hope it's just a temporary thing that's going on with you. Go bless you and your family, and I hope you find an answer.
D.