Headsets
I took my 5 year old up this summer in my Bucker Jungmann - she wore a David Clark (no active noise attenuation) with a cloth helmet to help hold it in place. It was a short flight, but the headset was adequately comfortable and fitted well in the open cockpit environment.
I'd agree that there would be no point taking very young kids flying as they cannot really understand what is happening and the "wonder of flight" is likely to be lost on them. I wouldn't take my 2 year old up - despite being utterly enamoured with aeroplanes, he'd have no interest in flying in one other than scheduled...and I'd be concerned about putting him off.
If however it is an A to B affair eg holiday, then why not...
I'm not aware that ANR emits anything harmful but I'm no expert. Once kids are 5 years old the concern about infant skulls is long gone...