Eating grass and weeds

I have a 9 month and 13 month old mini schnauzer. We have a big fenced in area for them to go to the bathroom and to play. We physically carry both in and sit there while they go to the bathroom. (We also discard poop every single time they do it, so none is ever left in the play pen, it’s instantly bagged and thrown away). But they will it stop eating everything in sight, grass, weeds, anything that’s in the playpen. They are never left unsupervised. While I grab something out of one’s mouth the other is behind me eating something else. It is non stop and driving us crazy. Then they go to the bathroom inside and get it everywhere. They act as if they are scavengers starving outside fending for their lives. Yet have food and treats inside. First dogs we’ve ever owned. Any advice to nip this in the butt??