gwraigty
Thinks s/he gets paid by the post
An issue would be initially opening the account, since fund companies typically require a minimum investment amount to open an account (e.g., $3,000 for many Vanguard funds).
So your child would have to contribute at least $1,500 to get started (and have at least $3,000 of earned income).
No. Forget Vanguard. Come on in to the 21st century. Schwab and Fidelity have no minimums to open a Roth IRA.
https://www.schwab.com/public/schwab/investing/accounts_products/accounts/ira/roth_ira
https://www.fidelity.com/retirement-ira/roth-ira
Both also offer no load, no transaction fee mutual funds with no or super-low minimums and NTF ETFs.