Triclopyr if used cautiously does not kill the grass. There maybe some affect but grass recovers. The information on the link provides information on how to use it. Fall application works best and follow up will be needed. I always choose treating just the weed to reduce the amount of herbicide used. I favor basal application. 
https://pleasantvalleyconservancy.org/sumac.html
For long term control of the suckers severe them each year by inserting a spade into the ground about 8 inches deep along the sumac bed. If that isn’t enough a barrier of metal like roofing flashing can be inserted vertically into the ground along the bed. It obviously is labor intensive but is a more permanent solution. 

Use Garlon 4 or Triclopyr 4, follow the directions for applying it on the label. One must follow the directions for the product they buy. All pesticides are regulated and the label is legally required. 
Basal application means you cut off the sumac close to the ground and apply the product to the cut stem. Or don’t cut it off but treat the bottom 6 inches with herbicide all the way around the stem. 

You are welcome. Sumac are beautiful especially in the fall but they can be a pain.