Adding talcum powder to paint sounds like an old wive's tale. I wouldn't do it.
Satin has a low sheen to begin with. Its not like a semi-gloss or gloss. I wonder why you object to it so much. I don't think most people could really tell a difference between flat and satin.
No matter. If you want a flat that is washable, then follow our standard advice. Buy your paint from a paint store. Get the best flat paint they have. Avoid big box and discount houses. Recommended brands: Sherwin-Willams, Devoe, Pratt&Lambert. There are some regional brands that are supposed to be very good, but I don't know the names (they aren't available in my area).