From the IRS point of view, you have to charge a minimum interest rate for personal loans, called Applicable Federal Rate (AFR)
https://www.investopedia.com/terms/a/applicablefederalrate.asp
The rate depends on the length of the loan term and the IRS updates this rate once a month. So, you need to look it up when you sign the paperwork (then it is locked in at whatever it is for the rest of the loan term). The AFR for a >20 year term was 1.90% in November 2021. Not sure the December rate has come out yet.
Of course, you are free to charge a HIGHER rate, if you want.
You will have to report the interest as income at tax time and your child will be able to deduct it from their income. You will have to send your child a form 1098 and report to the IRS respectively