Mostly in the US, I have noticed no announcement; gate staff move about the passenger waiting area and round up families with infants and young children – not those prone to line up for early boarding with “children” who qualify for UM status! – and pre-board them just after those needing special assistance (e.g. wheelchairs.) A welcome relief from herds of people wanting to preboard because they have four active children including two subteens and enough hand luggage to swamp the Titanic (the norm on WN, btw. I have seen twenty and more people preboard, some actually needing it.)

I recommend ANYONE expecting and needing preboarding let the gate staff know their needs.