Play music controlled by QR Code

> Is it possible to do that withe LMS?

yes

> Is there a Plug-In?

Not that I'm aware of.

> Or do you have any other thoughts?

Cool idea. Get your hands dirty :-).

You'd have to somehow identify an item (album/artist/track) by some id.
In LMS' favorites we use either the file's path (single track) or the
simplified title/name for albums and artists. That search term can be
found in LMS database in the titlesearch or namesearch columns
respectively. Yes, this fails if you happen to have two albums of the
exact same name (eg. "Up", "Best Of"). But I assume for the few albums
for your daughter this should be good enough.

Have the Pi recognize that identifier, send a "playlist play {url}"
command to LMS.

--

Michael