Assuming that the child uses this mortgage for business purposes, i.e. buying a rental property - the child has to send Mom form 1099-Int if the annual interest on this loan is above $600.
I think it may be lower, if one is willing to entertain an alternative viewpoint:
$10 rule is in Sec. 6049
Sec. 6049 (b) (2)
(2) Exceptions For purposes of subsection (a), the term “interest” does not include— (A) interest on any obligation issued by a natural person