If my daughter found a bent stick pin in the back of one of her cookies, what can I do?

If your daughter was not injured and did not require medical treatment, the case is not worth pursuing because little or nothing will be recovered as compensation.

If your daughter was injured and required medical treatment documenting her injury, when she completes her medical treatment and is released by the doctor, obtain her medical bills and medical reports.

Prior to filing a lawsuit, it may be possible to settle the case with the insurance carriers for the manufacturer and seller (store where the cookies were purchased).  Compensation for the medical bills is straight reimbursement.  The medical reports will document the nature and extent of your daughter's injury and will be used to determine compensation for pain and suffering, which is an amount in addition to the medical bills.  If the case is settled with the insurance carriers for both the manufacturer and seller, NO lawsuit is filed.  If the case is settled with neither defendant, name both manufacturer and seller in your lawsuit for negligence and strict liability which will be explained below.  If the case is settled with one, but not both insurance carriers, only name the defendant in your lawsuit with whom the case did not settle.

Your lawsuit against the manufacturer and seller would have separate causes of action (claims) against both defendants for negligence and strict liability.

Negligence is the failure to exercise due care (that degree of care that a reasonable manufacturer would have exercised under the same or similar circumstances to produce a product that is not defective). 

Strict liability is liability imposed whether or not due care was exercised.

Both the manufacturer and seller (store) are liable for negligence and strict liability.  The store is liable even if it could not have known that the product was defective.

If you are dissatisfied with settlement offers from the insurance carriers, reject the settlement offers and file your lawsuit for negligence and strict liability.  I assume your daughter is a minor.  You will need to be appointed guardian ad litem to file a lawsuit on her behalf because a minor cannot file a lawsuit herself.  If the case is NOT settled with the insurance carriers for both defendants, you will need to file the lawsuit on behalf of your daughter for negligence and strict liability prior to the expiration of the applicable statute of limitations or your daughter will lose her rights forever in the matter.