Their own contributions are Basis; that gets entered on Form 8606. You didn't state if they took more than this amount. Roth has distribution ordering rules. Also, there are three different 5-year rules (account of this type open at least 5 years, conversions left in at least 5 years) and there is code J for early distribution = under age 59 1/2. That means Form 5329 for penalties. There are some exceptions.

https://www.irs.gov/taxtopics/tc557