More kids, more hands.
Google isn’t telling me much about the origin or the history or the meaning of this expression in German. It was apparently the title of a 1976 propaganda piece from the DDR, but I don’t know if that’s where this comes from, or if that piece simply made use of an existing expression.
My guess is that it means that a family that has a lot of kids is better capable of running a household, or a family farm maybe? Using the kids as free labour? I don’t think there’s an equivalent expression in English if so, and “more kids, more hands” certainly doesn’t say much.