I'd suggest going through my program which answers all of this at
https://app.frum.finance/
But if you're looking for just a quick answer instead
1. Read simple path to wealth
2. Open a custodial brokerage (m1 or fidelity)
3. Invest in VTI, don't look, teach child to hold and not to sell, as they approach the date of needing the money, cash out.
Lots more details in the free resources though.