If the 24 year old is Not a dependent, (which I am assuming is not) :

1- check the recipient box for taxpayer or spouse , It really does not matter whichone is checked because that information is not trasmitted to the IRS. 

2- Go to the allocation of policy and allocate 100 % to the policy holder (parents), (you will see that the child return does not change the refund or the amount due)  or allocate any amount they both agree to. Usually, I put the 100 to the parents. 

Now, 

If the 24 year old is still dependent, (which I am assuming it is not) , the form goes in the parents' return. 

Thank you and have a great day.