I wonder how many times you have heard parents or adults say to children be nice, play nicely or be good. I also wonder how many of these folks have thought about what they are actually saying to the child. What do we actually mean when we say be nice, play nicely or be good. How can a child and further more a very young child know what being nice, playing nicely or being good actually means in practice.
Words like this are often said to a child when they are in conflict with another child(ren). To my mind it is far more useful and relevant to a child's development and growth to talk to them about what is going on and use words that will help the child to learn for themselves how to resolve conflict. We can find out what has happened, we need to gain both sides of the story, even if we were a witness this is important as we can never know how another person experiences a situation. We can then use expressions such as what could you do next, what can you do to resolve this and let the children sort it out for themselves rather than taking over and doing it for them.
I have found that the less I interfere in conflict between my children, the quicker they are now resolving this for themselves. If they come and ask for help or they clearly are not resolving things for themselves I do offer advice, but I don't take over and resolve the situation for them.
In my working life I found conflict with colleagues the hardest aspect of work to deal with probably, I now realise, because I did not build a skill set to deal with this as a child. The world we live in is, sadly, full of conflict it is important then that we give our children the opportunity to build and develop these skills when they are young.
To go back to what I mentioned at the beginning about the meaning of the phrases so often used, be nice, play nicely and be good. I would interpret them to mean if you do this, I (as the adult saying it) will think that you are good and love you for it, don't obey me and I will think worse of you and withhold my love or punish you. Not only is a child bewildered about what is being said to them, but doubly so by your reaction when they do not obey your commands.