Thanks for responses. Think I'll start with DiskUsage and Clean Master Lite, then reformat the SD card as internal storage and see where that gets us to. I'll report back.
In the event I told her about Clean Master Lite and she ran it and reported back 'Yes, it worked. But it only cleared about half a gig.' I pointed out this upped her available memory by 50%+...
I ended up running it on my own Samsung S7 edge, and it got rid of something approaching a Gb, which again doesn't sound much, but when that increases your available storage by - in fact - over a third, it seems like a no-brainer. Ingrate that I am, I immediately uninstalled it. I know where it is if I need it again.
Whether it sorts out my daughter's actual issues - performance and, crucially, an inability to install new apps - remains to be seen. I think she's hankering for a new phone in any case. Her younger sister recently got one, and....
But yes, thanks for the Clean Master Lite tip. In time I might look into the using SD for internal, but for the moment I'll see how it goes.