Five is young and a 5 year old's understanding can be limited. Your best bet is to pull him/her aside and explain that picking on other children is mean. Keep it simple and don't be mad at your child about it. Ask your child how he/she would feel if he/she was being picked on. Ask why he/she is picking on others. Maybe your child is going through something and is feeling helpless. Maybe before you have your conversation you could find some books about bullying and read those. Talk as you read. It is important to stop your child from becoming a bully early.