I don't know, it's true or not that every adult is a child at heart. If there is some truth in it then I think every human being must pay attention to it. They need to channelise their heart and brain properly. What else can be so interesting than drawing and colouring! It gives immense happiness as well as the proper involvement of brain, eyes and hand help in creating a co-ordination between each part of the body, which is very good for health.

What you heard is definitely right. There is a child in everyone of us. Bringing out that child not only enriches your life but many a psychological disorders can be kept at bay. One need not be an artist to do coloring  a picture. The very activity implies that you took up a new activity which makes you rejuvenated and creates child like enthusiasm in you. This is a must for all of us who wish to lead a blissful healthy life.

yeah everyone in his/her life have faced childhood and that experience no one can forget even he/she becomes adult or old .The childish behaviour remains in oneself till death

And this child in us has to be brought out sometimes. It helps in keeping you busy. This in turn  helps in many ways to make your health good that's free from any ailments.

I can't say about others but as I know myself, I am a matured person but I melt easily with true affection on any issue but if someone try to broke me,I am complete stubborn like a child. I tried to change it but only succeeded in moderating it.

By engaging in coloring, we become creative and increase our level of concentration. It helps to reduce stress and makes life comfortable. It need not be the art of coloring, but any creative work, where we can concentrate our mind combined with our heart, will relax us and relieve us from tension. Concentration is the key that makes every creative work a kind of meditation. 

Yes. it need not be coloring. Change of activity of any kind will be taken as a challenge by the mind and in its turn makes you concentrate on the job you undertook. This not only makes one develop concentration but rejuvenates your system. For this you should call back that child like enthusiasm..