I think it's worth it OP if you can do it very calmly and factually, especially if you think the parents are the type to care and to intervene. You could just say, 'I thought you might want to be aware that your child is doing X on a daily basis to my child. He seems to find it entertaining but to my son it is bullying. Do you want to sort it out with him, because if it doesn't stop today, and for good, I will have to get the school involved.' You can tell a lot about the parents from how they'd react to something like this. But there's usually a reason children bully other children, and that reason often starts at home.
I called some parents once who were bullying DS2. Two sets of parents, who were close friends and their DC were close friends who picked on DS for entertainment. One mum listened, said thank you for letting her know, she'd deal with it. I found out later she'd told her husband and he'd got really angry with his son - shocked that his child could be so cruel to another child.
The other mother said nothing - literally nothing, when I called. When I saw her at school events she just smirked at me. Her son was later expelled for, among other things, selling drugs.
I was interested that the parents who dealt with it helped their son realise what effect he was having on others, and the parents who didn't ended up with a little shit of a drug dealer on their hands.