How can we properly teach kids not to mock or sometimes bully other children who are disabled without sounding mean?

First, you have to let the kid understand what bullying is, and that you will not tolerate any kind of bullying. This will start by teaching the kids how to respect and be kind to others regardless of their condition and status in life. It is also important to teach kids how to socialize with other kids to prevent bullying among them. Lastly, be a good example to your kids and encourage them at all times to practice good behavior. There are more tips on how to teach a child not to be bully. You may check out this link: http://kidshealth.org/en/parents/no-bullying.html