Letting baby watch TV

Suzanne
I just went back to work full time and my parents are the primary babysitters. They do it for free so I really don't want to complain and I'm looking for ya'lls perspective. 
My baby just turned three months, when I got home from work the bumbo seat was on the couch and cartoons were turned on. My baby was getting changed in the other room. So I asked about the bumbo, was he sitting in it on the couch? Yes but only with me sitting right next to him! I explained their dangers (had a friend with a baby who cracked their skull) and asked that that only use it on the ground. Their response was that they would "never let him fall".
Then they started telling me how funny it was that he loved and watched the tv so much. I have never turned cartoons on for him. To be honest I don't want him watching tv! I didn't want to get upset if I was just overreacting so I just said something like he may like them but I'm not sure I do, he seems too young. 
What would you ladies do?
Do you let your baby watch TV? Why or why not? If so, what and for how long? 
I so appreciate them and their help so I don't want to be bratty or demanding but I'm not sure I'm happy with their babysitting style and it's only day one.