Unfortunately for you, you do not get to decide when to file the application for the child nor do you get to pick the representative payee. The custodial parent is generally picked to be the payee.

You could jump the gun and file the child's claim and payee application first, and provide evidence that the mother should not be payee. But a parent with custody has priority unless there is some compelling evidence (documents, not words) that indicate she should not be payee. You may not win this.

If the claim is not filed until your child is 18, then the 18 year old is generally paid directly, but by waiting, she may lose some retroactivity if you wait too long.