assalam wa alykum
is it permissible to make the small kids wear the dresses of spiderman, batman etc. just to make them happy?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

We previously clarified in Fatwa 87564 that the preponderant opinion is that it is permissible to allow the children to wear clothes which carry some pictures because these pictures are not respected; rather they are treated with disrespect. This is in principle, however, it should be noted that letting the children wear the clothes mentioned in the question may involve violation of Islamic rulings. That is because these cartoon characters sometimes do practices that are not approved by the Sharee‘ah. Wearing the clothes which carry pictures of these characters may make the child attached to them and hence imitate their practices. So, it is preferable to completely avoid dressing the children in these clothes.