Everything in moderation is a good guideline. Health issues aside, if kids get fast food all the time, it isn't a treat, it becomes daily fare. I think fast food is okay on a very limited basis. If it is done on a rare occasion is can be a special family night or treat.
The only thing I can think of to do is to teach kids about nutrition so they make better choices and you don't have to be the bad guy. Also if meals are planned ahead, a family can cook every night. Just sit down with the family and make a plan. The kids can prepare, peel etc. the veggies, Mom and dad can toss some chicken breasts in the oven and make a pot of rice. Dinner is done and everyone worked on it together.
There's no reason that working parents should have to do everything alone, kids can help. They eat too, so why not?
I don't think overusing fast food restaurants makes one a bad parent, but it certainly isn't what is best for the child's overall health.