I've never hunted Grass Lake specifically. Is it even huntable? I seem to remember it being largely encircled by cottages and homes. Be aware that just because a lake has a public access/launch doesn't mean it is open to public hunting. You need to be hunting the lake adjacent to public land or have landowner permission. Might not be any pressure if you have permission from a landowner on a lake with no public land.
Opening weekend will be tough wherever you go. I have had some luck hunting smaller pothole lakes in the area of bigger lakes like Grass Lake. When the ducks get tired of getting blasted at on the big lake they will look to the smaller lakes in the area. A small spread with 1/2 dozen decoys will look pretty good to them. I've never gotten a limit this way, but can get a few ducks on a good day.
After the first few weekends and some good cooler weather, I would suggest hitting some of the managed areas. These are generally pretty accessible and will get you and your kids into some ducks.
If your kids are old enough and you have the boat for it, you aren't too far from Lake Erie and the Detroit river for some diver hunting. Weekends in late October and November will be busy, but there is room to spread out. It goes without saying that boating on the big waters in the fall can be dangerous. Make sure you and your boat are well prepared if you want to hunt the big waters.
