I have an 84' Nishiki Cresta I believe, but a road version meaning it doesn't have cantilevers. It can haul a decent amount of weight, but the max I've put on the back rack was about 50 pounds. It handled it and nothing broke. However, I have to say I thought the frame was a bit soft with just me on it, but with 50lbs of sound equipment (let alone your child) I absolutely would decline because the thing felt like a noodle.

My Surly Travelers Check with those S&S couplers, which I hear makes it stiffer, handles weight like that all day without complaint and feels oh so poised.

Would this perhaps be a role best suited to a MTB re-purposed to a touring rig? I only know my Surly and my early 80's Cresta so please keep that in mind. I'm comparing an early 80's noodle and something rather modern with oversized tubes.