I'll let competent players play my guitar for a tune or two, and I'll let almost (note the qualifier) any child hold it and twang the strings -- any child under 11 or so. Most really small children are more respectful of instruments than are adults. If a child is too small to hold up the instrument (2-4 yrs old), then I hold it for them, make chords and let them strum. It always gives me a little thrill to see tiny fingers tremble as they touch the strings, and their little faces light up when they can actually produce a sound.

As for tuning it - well, if a good player thinks it's out of tune, then let 'em go for it. If it's still out, then it's their problem. When I get it back, I'll just retune, no harm, no foul.

Need to add, though, that I just play on the porch with friends, for my own amazement at home, and to amuse kids. If I were professional, I might have other feelings.