If they play the piano, they are likely to end up playing lots of depressive Chopin and defiant Beethoven and fantastical Schumann and suicidal Schubert. Most of the piano literature does not have what I'd call a properly calibrated Catholic ethos.
I have some whacky theories on music, because I listen almost exclusively to French Baroque Catholic music and medieval and Renaissance polyphony ( with a bit of chant ).
But I see the piano as expressing a kind of humanistic spirit, it is usually used to translate fleeting emotions. But if you listen to polyphony like Palestrina, there is no human emotion, it's all about giving glory to God. It's not about "Oh, I'm sad today, I think I'll write a sonata so the whole world knows I'm miserable." I think the devil used music in order to make people enslaved to nostalgia and romantic feelings and morbid sentimentality, etc.
I agree that learning an instrument develops the brain, though, the same goes for foreign languages. The best instrument for Catholics is the voice, but you can't buy one of those.