The answer is maybe. It's not only a question of whether or not malpractice was actually committed but whether or not the resulting damages render a malpractice action economically viable. Gather your records and consult with an experienced malpractice attorney as soon as possible.
The answer is maybe. While the building's insurance may provide medpay coverage, they are under no obligation to offer it. It may be the right business decision for them to do so, but doing the smart thing is not always in a carrier's wheelhouse. Consult with an attorney in your area before deciding on a course of action.
Although there is no question, it can be assumed you are inquiring as to whether or not you have a viable claim. You should consult with a personal injury attorney as soon as possible as there may be short filing times involved. I'm sure there is a lot to this story which really requires a face to face consultation.
If you are the owner or an owner, under New York law you could be passively liable for the injuries or damages caused by your friend. Make sure you report any accidents to the insurance company covering the vehicle at the time of the accident. If timely reported, you'll be covered and they'll deal with the claim.
Is this a case you're handling yourself? If you are, you shouldn't be. If you have an attorney, he or she should have put the manufacturer on notice and investigated the possibility of successfully pursuing a products claim. Make sure the sink itself has been preserved and is available to determine why it failed. If your attorney is not looking at this aspect of a claim, perhaps you should consider a second opinion.
You are not "charged" in the criminal sense but you are liable in money damages. If you receive a letter or legal process concerning a particular incident, turn it over to your insurance company immediately. If there was an accident, make sure your carrier is aware of it. Not telling them in a timely fashion may result in the insurance company denying coverage in which case you may become personally liable.
Make sure she gets the medical attention she needs and all relevant insurance companies are put on notice. The statute of limitations for her is tolled until she reaches the age of majority. If something accident related develops in that time, you can always proceed with a claim. You do not indicate if the vehicle was private or public. If the latter, her age will not toll the time to file a notice of claim in which case you should seek the services of an attorney as soon as possible.