Social Security Disability

My husband and I are seperated, we have no legal seperation agreement. He is on Social Security Disability, my kids get benefits from this. I have sole custody of my kids, he only sees them maybe every couple of months for a few hours, ususally at my home. My question is, can I have these benefits paid directly to me cause the kids are with me, every month it is a struggle with him to get this money. I am just tired of fighting and worrying each month.
Also, if I write a letter stating the amount he is willing to pay me each month, and we both sign and have it notarized, will it become a legal document for me to use in court to have that money paid directly to me.

Someone told me that if the money he has given me isnt stated on the check as child support, he only gives me cash. I can sue him for back support. Is this true?

I believe he is being advised by the woman who ruined my marriage, and brainwashed him into leaving.

Thanks!

You can get child support in arrears from the date of filing your child support action. You may also show actual expenses for time prior to filing, and the court may make an award for the time before filing. He would receive credit for any money he gave you.