How much ipad time do you think is OK for a preschooler? My boy has learnt a lot about topics that interest him such as dinosaurs through YouTube videos. I'm just thinking whether is it causing more harm than good?

Moderation is the key. Parents have to step in to control the time for ipad. I only offer ipad/smart phone when necessary e.g. fussing in long car journey. Nothing beat hands-on learning through real experiences and interactive learning. Ipad is a form of passive learning and long term usage will detriment kid's attention span. Also, high chance kids will develop myopia if use ipad for long term.