Most bus drivers are angry bastards, and I wouldn't be surprised if they didn't let a child on, considering the ones I've had...but I'm still nice and say thank you, because generally they have a shit job.
Today, one of them 'inspected' my bus pass for like five minutes, he was just staring at it. I don't think he was even reading it. Dyslexia, presumably. I was like okkk then. This is awkward. Because everybody on the bus just stares. People always watch other people getting onto the bus. It pisses me off. Look outside or at your lap.
Anyway. /rant
Morally and ethically, a bus driver should/would normally let a child on. There definitely would be a duty of care factor involved too.