Easy fix, keep them away from the grass by making sure they are leashed and under your control, or keep them in a toileting area by using a barrier fence (like a puppy pen).

Puppies tend to get very distracted! LOL

Be sure are feeding a quality diet not something from a grocery store that has a lot of fillers or dyes. It could be that they need some additional nutrients.

Grass eating is nothing to cause concern unless you have used fertilizers and pesticides.

Drop by a local pet store (not a superstore — try an independent) and they can help you select a better diet. Just be sure to change over gradually over a two week period to avoid any intestinal upsets.