How much can you sue for assault?

I agree with Artemis' answer and am responding only to your Additional Details:

It does not matter if you have a contract for the purchase of the phone signed in the girl's blood and witnessed by a judge. Contracts with a minor are void. They can not be enforced. It does not matter if you have proof she was going to buy the phone. She is a minor. She can't be forced to buy anything.

No. It would never constitute stealing because your sister voluntarily handed her the phone. She didn't steal it from your sister. At best, she is wrongfully keeping the phone. Your mother can certainly ask a judge to order the return of the phone - but if the girl claims she doesn't have the phone or even if the phone is broken, your mother has no recourse and no ability to get any money.

I don't believe that a legal aid attorney who was actually licensed to practice law would ever tell you that you could or should name the business owned by the father of the girl who assaulted your sister. Every first year law student has to take Civil Procedure. And every law student learns about "personal jurisdiction." This means that a plaintiff can only sue a defendant who is responsible for the bad act. In this situation, the girl is definitely responsible. Her parents MIGHT be responsible - but this depends 100% on your state's law and the facts and circumstances. Under absolutely no circumstances would the business ever be a viable defendant because the business is not responsible for the actions of the girl who hit your sister. And if you name a defendant improperly, you can find yourself hit with a counter-claim for damages associating with tarnishing the business' reputation.

Your sister is a minor. Your mother is required to provide her with shelter, food and all other necessities. Because your sister is not expected or required to support herself, she cannot sue anyone over her inability to get a job.

Unless you are your sister's legal guardian YOU do not sue for anyone.

Your mother is the only person who can file a lawsuit.

I think your mother will be completely and utterly wasting her time and the time of the lawyer. This is a small claims case. Your mother can sue for the statutory maximum allowed in your state but I don't see any lawyer even being willing to pursue this matter.

First, a lawyer will only take a personal injury case against a defendant they believe they can collect from. Depending on the age of this girl and her history, it is possible only this girl can be sued. But unless her parents have a great deal of money, assets that make it likely they will be able to pay a judgment.

Second: Since your sister precipitated the fight by trying to collect on an invalid agreement it is very, very possible there won't be the potential for an award for pain and suffering. (The idea being that there are consequences for bad decisions.) This could mean your mother might only be entitled to her actual out-of-pocket payments on the medical bills. And there is no doubt your mother should be entitled to collect for what she has to pay on the medical bills for anything associated with that final punch.

Since attorneys generally take a 33 - 40% contingency fee plus their costs and even under the absolute best of circumstances I don't see this case generating more than around $15,000, I don't see an attorney being willing to invest that much time and effort over a case that cannot and will not generate more than $5,000. Particularly since the girl (when she turns 18) or the parents can simply go to court, file for bankruptcy and wipe out the judgment without your mother ever seeing a penny.

No. The cost of the phone can't be sued over. Your sister and this girl cannot enter into a legally binding contractual agreement over the sale/purchase of the phone. It isn't enforceable and I don't see how it can possibly be a compensible damage.

If your mother came into my office with this issue, I'd advise her to sue the girl and the parents for $5,000 in small claims court. And I'd remind her that getting a judgment (winning the lawsuit) does not provide a guarantee of payment.)

EDIT: NO. Under absolutely no circumstances could your mother EVER name the father's business in this. The father's business had nothing to do with the assault.

there's a statute of obstacles for each state. likely definite. yet when the police did not prosecute maximum trials would look on the data that led them to no longer prosecute. perchance there wasn't sufficient data, or there have been conflicting memories, or some thing else? further you want to tutor harm from the attack? clinical institution expenses, psychologist expenses, and so on.