Small claims court is a simple way to litigate money claims, as this would be. One side would argue that the other party owes them money; the other side would dispute the claim. In general, a person does it have the right to damage another person's property, so if B damaged your property, you might have a valid claim. The advantage of small claims court is that it's quicker and cheaper. However, one may need to hire an attorney anyhow, in order to correctly present your claim that B should be made to pay.
If you can prove that B maliciously kicked your backpack in order to cause you damage, the matter will probably be resolved easily in your favor. More likely, this was an accident (that was what you thought), so now the question is whether this was negligence on their part, or on your part. B's negligence could have been in not using ordinary care when walking, your negligence would be in putting the backpack in the walkway. The judge would listen to your explanation of the surrounding facts, and then lay blame. In Wisconsin, the judge would compare the contribution of the two parties and come up with a percentage. In this situation, B would not have a basis for counter-suing you, which simplifies the matter a bit. If you more than 50% responsible for the damage, you cannot recover from B. If you are found to be e.g. 49% responsible and the defendant is 51%, then he is liable for 51% of the damage that you have suffered.
If you consult with an attorney, they can give you a good estimate of the likelihood that you'll lose on the grounds that your negligence was greater than B's.
