Can I defended myself if attacked by minors and to what level can I restrain if I feel threatened?

You can only defend yourself if you are in physical danger. If you are not attacked, you cannot attack. I would advise talking to the bus driver or just moving to get away from the kids who are swearing. It is hard to avoid swearing in public.

In order to claim self-defense in a situation a person must do what is reasonable under the circumstances. This means that you need to have to reasonably believed that you were in imminent danger of being killed, injured, or touched unlawfully, reasonably believed that you needed to use force to prevent that from happening, and used no more force than was necessary to prevent that from happening. If in your situation, a reasonable person would feel that all the above requirement were met you could use self-defense. Age does not matter. However, could you reasonably feel threaten by a four year-old as an example.

You have a right to self-defense, regardless of whether the perpetrator is a minor or not. I'm a little unclear about what you mean by how far you can "restrain" if you feel threatened. If you're asking how far you can legally go to defend yourself, the answer is that you can do whatever is reasonably necessary to defend yourself. So, for example, if someone assaults you, you fight back, and they then retreat, you don't have the right to continue fighting them. Also, you can't "over-defend" yourself. If someone slaps you and you pull out a gun and shoot them, that's not self-defense. Bear in mind, that these are very basic hypotheticals. Every case involving self-defense is going to be different.

You can defend your self or others if physically attacked. You need to use force appropriate to the circumstances. You can only use deadly force if there is a fear of death or great bodily harm.

If someone physically starts to attack you, then you are entitled to use force to reasonably defend yourself again such an attack. You cannot use excessive force meaning you can only use the force that is necessary to stop the aggression and you cannot be the one who initiated the physical assault. You cannot use physical defense to combat offensive words or racial slurs.

The actions which you have described are violations of the law, namely racial intimidation and common-law assault. It is illegal to place anyone in fear of physical harm or death. You have a right to self-defense and are allowed to use reasonable force in order defend yourself (and your child). I would suggest that the next time this occurs you should, you can take a photograph of the perpetrators and call 911.