While I understand the concerns, a statistical link between former footballers and increased risk of dementia is hardly an indication that heading is the cause. There are other things affecting footballers that generally don't affect the majority of other people, increased intense physical activity as an example.
It's only for training sessions. As a junior coach for many years Ilm more interested in ensuring they can use both feet. When they reach around 8 they want to start heading the ball. That's when you get the size 1 and 2 balls out, let out a bit of air and get them to practise technique with that. We do it a couple of times a season for less than five minutes. If they can play with both feet and make quick decisions they rarely need to head in a match anyway. My view about it is influenced by heading an old, wet ball as a kid because for some reason in the late 70s our school still had brown leather laced balls that weighed a ton. They were in stark contrast to the white captain's balls or orange trophy balls that we used at home. The school footballs used to leave you dazed sometimes and, at very least, rubbing your head.