It is not "under the age of 18". It is no older than age 16 at the end of 2020. They had to be born in 2004 or later to qualify.
The IRS phone center is closed but will probably re-open soon.
The IRS FAQ says:
https://www.irs.gov/coronavirus/get-my-payment-frequently-asked-questions
For security reasons, the IRS plans to mail a letter about the economic impact payment to the taxpayer’s last known address within 15 days after the payment is paid. The letter will provide information on how the payment was made and how to report any failure to receive the payment. If a taxpayer is unsure they’re receiving a legitimate letter, the IRS urges taxpayers to visit IRS.gov first to protect against scam artists.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**