"The ability to self-regulate emotions and behavior in response to changing environmental demands is among the most fundamental skills that children develop in early childhood and these skills underlie successful development in multiple domains across the life span."
http://www.apa.org/pubs/journals/releases/dev-dev0000536.pdf
As it is important to teach children emotional regulation it is also important to let them discover ways to practise those self-regulatory skills themselves. I think one of the biggest factors in successfully teaching emotional regulation is the adult's example.
Parents shoud be very careful especially when expressing negative emotions. I definitely think that those emotions should also be expressed. But the expression should align with the experienced harm so that children learn to estimate how to react properly to minor setbacks.
Negative emotions are and have been important for our survival. Thus, we tend to pay more attention to negatives than to positives. This is why we should knowingly pay attention to positives in our daily lives.
