Letting four year old drink coffee?

So I have a friend who has a four year old son. She works late usually so her father watches him. Well her father comes to this restaurant to drink coffee every. Day. I know this because I work there too. Well he usually brings his grandson but he lets his grandson drink coffee with him. Not just one cup, multiple cups. We have free refills so they will sit there for 1-2 hours constantly refilling. I didn’t know if she knew he did this so I kinda just brought it up like “hey you let ** drink coffee?” And she said of course that’s it’s perfectly fine and what not. But I thought coffee would be unhealthy to children? My little brother used to sneak my parents and they’d freak about how bad it is. Were my parents just overreacting? Cause now I’m not sure. My aunt decided to give my one year old coffee while she was babysitting and I wanted to flip shit but my same friend told me I’m overreacting. Maybe it’s just cause I personally hate coffee lol