Is it normal for kids to be sadistic toward babies

Worst is when they don't grow out of it by not developing empathy.

They'll regret it one day..

Not sure how old they are but I don't think it's abnormal if they're very young. You should inform their parents so they take note and are more watchful of the group when they're together.

Young kids may not understand that the baby is a person like them because it is younger and less developed. They may also find the baby irritating because of the sounds it makes, such as crying. In addition, children in general tend to have less compassionate and cognitive empathy.

What kind of cruel things?

No. They're displaying red flag behaviour. There's something seriously wrong with them if they think it is okay to be mean to a defenceless baby.

IMO children who act like this have something missing in their make up. They are deficient in some basic human quality, and will most likely become a serious threat to society, if allowed to live.