The define-record-type procedure is from SRFI-9. The #:parent propery applies to make-record-type which is a lower-level Guile specific. The define-record-type does not accept a #:parent keyword.
There is also a define-record-type in R6RS which has a (parent ...). See https://www.gnu.org/software/guile/manual/html_node/R6RS-Records.html
If this is confusing, it's because after R6RS was published, Scheme splintered and grew in different directions; Guile lisp supports all these directions at the same time. You can read http://dpk.io/r7rswtf if you are interested in the history.