Pay no attention to the “life-is-hard” salary jockey types above. Unfortunately, there is no online forum for private helicopter pilots, so we mix in on forums/groups such as this. They don’t seem to understand that helicopters can be used for fun/pleasure.
Both of my sons have been flying in the helicopter since 3 months old; they’re now 9yrs and 15yrs respectively and still enjoy flying. Up to around age three, they sat in the back (of course in cars seats of varying sizes) with Mummy attending. From age 3yrs, they’d sit up front with me. From around age 5yrs, I started taking them out with their friends and have flown many children over the years.
At around 18-20mo, they don’t want anything on their head and may try to take it off (like a sun hat at the beach), but this will quickly stop and not be a problem going forward.
At that age they always nod off asleep and you can fit the headset on them once asleep. Also everyone else aboard can make a fuss about putting on their own headsets – they like to emulate actions. Also, tilt the mic boom up and out of the way so they can’t see it. You could even connect some music to their headset.