Hi Shirley, I see from your message history you are new to genealogy.Well, welcome to lots of ups and downs, but also lots of fun. (^_^)
I tried some indexes, but Warren is a fairly common surname, and given names of Richard and Robert sure don't help you.Rather like many of my ancestors.Since you are looking for info on your grandparents, you may know when and where they died.If so, you can obtain a copy of their death certificates, which should show the names of their respective set of parents.Accuracy only as good as the knowledge of the informants, but a copy of their application for Soc Sec cards was filled out by them, and should have names of parents, too.
Here are some sites you can try, especially if youhave a middle name, or initial, for Richard.
If you find Frank & Amelia on SSDI and you decide to send for a copy of SS application; on the far right side, click on "SS-5 Letter" it will give you form you can print out, and it will have the name and SS number of the person youhave clicked on.
Here is TXGenWeb site, click and look at everything, then scroll down to TX County Index option, click on "D", then find and click on Dallas Co.See if they have anything on town of Lancaster.
The Dallas Public Library has a great genealogy collection, and if you cannot visit there. See what is on the county site, maybe the library catalog is online.Be sure to post your query on the Dallas Co, TX GenWeb site, too.
Also, be sure to talk to any and all family members that will give you the time.Be forwarned not all family stories will match exactly.Time does play tricks on our old memory cells.Happy Hunting.