Anything you and the other side agree to regarding the rights to publish, release, modify music, on the one hand, and the payment of fees or royalties on the other, is legal. You nd the producer can literally agree to essentially any way to handle this that makes sense to both sides. But you will be held to exactly what is in the written agreement or contract. If the contract is badly written, that may hurt your grandchild's interests. Since we assume that you are not familar with copyright law, musical licenses, or payment of royalties, all of which can be very technical and confusing (and so contain a great many opportunities for you to make a mistake in negotiating or drafting an agreement), you are *strongly* encouraged to let an intellectual property attorney or lawyer who handles musical artist contracts help you.