One of two things would have happened when the child was admitted. Most likely is, the parents were required to assume legal liability for the debt. Most unlikely is, the hospital omitted that and either failed to get anybody signing the financial responsibility form, or they had the child sign it. In any scenario, there is no contract between the hospital and the child (though before leaping to that conclusion, you should name your state and probably the child's age, since the "minor's can't contract" idea is set at the state level). Since there is no contract, there is no debt. There is a federal law about collecting debts, 15 USC 1692f, which prohibits a debt collector from using "unfair or unconscionable means to collect or attempt to collect any debt". In particular
(1) The collection of any amount (including any interest, fee, charge,
or expense incidental to the principal obligation) unless such amount
is expressly authorized by the agreement creating the debt or
permitted by law.
Since there is no agreement, they may not collect on the putative debt. Now, it is legal for the hospital to attempt to collect the debt, given the definition of "debt collector":
any person who uses any instrumentality of interstate commerce or the
mails in any business the principal purpose of which is the collection
of any debts, or who regularly collects or attempts to collect,
directly or indirectly, debts owed or due or asserted to be owed or
due another. Notwithstanding the exclusion provided by clause (F) of
the last sentence of this paragraph, the term includes any creditor
who, in the process of collecting his own debts, uses any name other
than his own which would indicate that a third person is collecting or
attempting to collect such debts.
In other words, creditors (the hospital) aren't subject to the regulations that debt collectors are subject to. I assume that by "sent to collections" you do mean passed on to a debt collector.
Having said that, there is also a possibility of the hospital suing the minor for "unjust enrichment", since you don't need a contract to owe money for services rendered (esp. if you are unconscious and incapable of agreeing to treatment). But on the third hand, parents have a state law obligation to provide necessary medical treatment, so the parents may owe that amount, under the doctrine of necessaries. Your attorney would be able to bring together the relevant facts about law in your state and the particulars of the case (nature of the procedure, age of the patient, actions of the hospital and parents).