Your wife can unenroll at the IRS web site.
Unfortunately, the function that would allow you to enroll your child to get the advance payments in your name is not available yet. The credit will be reconciled on your tax return and if you claim your child as a dependent, and did not receive the full amount of the advance payment, you will get the credit on your return. The advance payments don't change the amount of the credit, just when it is paid.
In order to claim the child as a dependent, the child must live with you more than half the nights of the year, or if the child lives with the other parent more than half the nights of the year, the other parent must give you a signed form 8332, which you must mail to the IRS after e-filing. The full rules are here,
https://www.irs.gov/pub/irs-pdf/p504.pdf
