Looking for ideas on how to get my husband from adultery in California.

I have been married with him since 1978. Then I find out he has 2 other kids in the Philippines. He's a US citizen and already file for social security so that the kids can receive money from the government. He's trying to gain custody of the kids from the Philippines and talking to the kids mother to release custody of the kids to him so that he can collect the money that social security is providing to the kids which is ($800). How can I stop him from doing this, and where do I start? To me this is consider a fraud, do you agree? Do the kids have a right to become a US citizen?

How sure are you that he hasn't divorce you yet?
If he is an American Citizen and he divorce you under US law, he don't need to inform you about it, just to be mean. Technically if he did that, he is divorce to you under US law but you are not divorced or annulled to him under Philippines jurisdictions unless he forwarded the divorce papers to NSO and file for recognition, you should have the copy so you can clear your link to his name with NSO.

Admittedly, what he is trying to do is fraud, but if he has declared those children from the beginning of his application, this will not be regards as fraud. if this is the case those children do have rights to be US Citizen. But in order to do this he must have declared the other woman as his wife too!   
Unfortunately, Infidelity is not a serious crime to them. You may file a case against him but it will be buried and forgotten especially if you are from the other end of the world, unless you made an effort to go to US and pursue it while he is still on process of his applications you may still have a chance to stop the processing if enough evidence is supplied. If this applications has been granted already, there's no way the US government will revoke their US citizenship or passports I'm afraid!