R.K.
Okay you watch Law and order SVU maybe his mom watches Sex In The City.
I would be concerned only because my daughter was grabbed on her boobs in 3rd grade and it continued for a year until I found out about it and filed a police report and then signed my kids and me up for Tae Kwon Do.
I would call the bus company and speak to a supervisor and ask that the driver keep her in the front seat and keep an eye on her and the little boy. He may just like her and think it's okay behavior. He may think of her as a baby sister and gave her a brotherly kiss. Ask that the driver call you at the end of the week and let you know what he(she) observed. You can call the princpal at the school but that may open a whole can of worms, needlessly. If the driver sees anything else then step in and speak to the princepal. I wouldn't talk to the boy's parents, I would let the princepal or school counselor handle that.