As far as I know, psychologists emphasize kissing children and likewise believe that is has a very positive effect on kids. I wonder what the viewpoint of Islam is regarding this issue? Is there any hadith concerning this matter?
It is permissible (جائز) out of mercy and compassion.
It is forbidden (حرام) out of lust.
Hadith 225, Riyad As-salihin:
وعن أبي هريرة رضي الله عنه قال: قبل النبي الحسن بن علي رضي الله عنهما، وعنده الأقرع بن حابس، فقال الأقرع: إن لي عشرة من الولد ما قبلت منهم أحدًا. فنظر إليه رسول الله صلى الله عليه وسلم فقال: “من لا يرحم لا يرحم” ((متفق عليه)) .
The Prophet (صلى الله عليه و سلم) kissed his grandson Al-Hasan bin 'Ali (May Allah bepleased with them) in the presence of Al-Aqra' bin Habis. Thereupon he remarked: "I have ten children and I have never kissed any one of them." Messenger of Allah (صلى الله عليه و سلم) looked at him and said, "He who does not show mercy to others will not be shown mercy".