If my ex had child support reduced because he lost his job but I found out he has been working again, can I get retroactive support for the kids?

The amount of child support is based on income.  You can request an increase in child support since your ex found another job and the increase in child support can be retroactive to the date the new job started.

To request a modification of child support, file an Order to Show Cause (court form) and your supporting declaration signed under penalty of perjury stating the facts in support of your request for the modification.  File these documents with the court with a proof of service and serve a copy of your documents by mail on your ex.  When you file an Order to Show Cause, the court will schedule a hearing on this issue.

You can either use a court form proof of service or you can write your own.  If you write your own proof of service, it just says that you are at least eighteen years of age and the attached documents were sent via first class mail unless stated otherwise to ___________ (name and address of your ex) on __________ (date).  You sign and date the proof of service at the bottom.  The date you sign should be the same as the date of mailing and the same date you file your documents with the court.