If you had yet to enroll your child in a Trump Account, then Treasury has already done that on your behalf.  This is how you claim your child's account vs someone else fraudulently doing it:

Download the official Trump Accounts app to claim your child’s account. During the claiming process, you'll verify your identity, confirm your relationship to your child, review your child's information, and accept the account terms. You can also claim a one-time $1,000 seed contribution from Treasury if your child is eligible.  Go to https://trumpaccounts.gov/