At 15 months old, my newest son is just about old enough to come out riding now, we think. So, we bought him a child seat, and a helmet, mounted the seat on our tandem, and away we went. Trouble is, the helmet has significant rear overhang, and this hits the backrest of the seat, making it rather uncomfortable for him. Things got worse when he fell asleep, and his head either lolled forward, or if it went back, it pushed the helmet over his face (which I suspect means it needs to be a little tighter too). We had to turn back in the end after only a few miles, because he was very uncomfortable. Pics below show the problem - how can we get round this? We can't be the only ones struggling with this!