When they are upset and need some time to calm down teach them how to do that and have a "happy heart" before they come back to the activity going on. Like this. Focuses on them and their feelings and helps them recognize.
How to Potty Train Early. She has some helpful tips. If your child can pull their pants up and down or takes them off when wet, and is interested in growth activities, its never to early to start them. Best advice, never push them, always make it fun and assure them accidents are OK. Their bladder will make its own schedule :) Plus underwear is much more comfortable than diapers!