As a non-violinist (as is DH), any tips on how to keep DS1's violin in tune over the summer holidays? Any app recommendations? Am I right to think it's hardly worth practising if it's badly out of tune, as he'll get the wrong idea of the tune in his head?
It's not so much that he'll get the wrong idea of the tune, it's that if he places his fingers in the 'right' places for the notes it'll sound all out of tune, and if he tries to make it sound right, he'll be putting his fingers in the wrong places and developing the wrong muscle memory for where the finger positions are.
I haven't tried the app, but you can get automatic tuners that clip to the string - not perfect but good enough. Longer term, you don't need to be a musician to tune a violin well enough, I would ask his teacher for tips on how to do it.