Breach the contract

What do you mean "enroll"?  If you child is a senior, he/she enrolls into a college this Sept, not now.  Now he/she is not enrolling, he/she is just deciding which college.   Are you using enroll to describe the admission process?   I assumes your kids is admitted by two colleges, and have not enrolled in either one of them (such as register a course.). 

First, your child and you signed a valid contract to attend the first college.  Contract can be breached.  It is a business transaction.  Breaking contract is not immoral.  Business breaches contract all the time when the deal is no longer desirable. 

Now if you want to backout, you have to breach the contract.  The first college will NEVER AGREE to release your child of the obligation he/she already entered.  They don't tell you what to do, because anything they say would be taken as an agreement.  You ask them:  "how can I back out of the decision?",  If the School says: "You can break the contract.", later you can claim THEY TOLD YOU TO BREACH the contract, and ask for your deposit back since THEY TOLD YOU TO BREAK it.

So if you want to go to the second college, you breach the contract with the first college, and lose the deposit, that's how the system works.  

Simple as that.