With the help of the LiederNet Archive, the following information may be found.

"Wenn fromme Kindlein schlafen gehn" does indeed appear in the work of Catharina van Rennes.
Her setting appears in
Lebenslust: Sechs zweistimmige Kinderlieder mit Pianofortebegleitung, the Composer's Opus 15, no. 3, published by Breitkopf & Härtel in 1892.

German composers who have set this lyric to music,
most often give it the title
"Kinderwacht" or "Kinderwache," for example:
F. W. Abt,
Karl Hirsch,
Julius Maier,
Robert Radecke,
and both van Rennes and Schumann.

In the latter half of the 19th century,
when these versions of the song seem to have been
most widely circulated and most popular,
the publishers, editors, and composers
themselves gave contradictory information about the source of the text.
One publisher maintained that it came from Des Knaben Wunderhorn.
"Volkslied" is not an uncommon label in song settings.
A few people did indeed credit Friedrich Güll,
and at least as many others, if not more people,
credited
Melchior, Freiherr von Diepenbrock as the Author.


The source located by Grishka for the text,
credited to Güll, is not a bad source.
An even better source would have been
a collection that is all Friedrich Güll,
for example his
Kinderheimath in Bildern und Liedern from 1836,
which may be scared up at Google Books.
But this text is absent from that collection -- I looked.

Nor could I find this lyric in Diepenbrock's work.

Editors such as Franz Magnus Böhme
(Deutches Kinderlied und Kinderspiel)
write that, in 1849, an issue of
Fliegendes Blatt or Fliegende Blätter
first printed the song.