Although it seemed like a kind gesture from a caring uncle, Kamsa's invitation was actually a trick by the cruel king who wanted to kill the boys. Kamsa's invitation was actually a trick by the cruel King who wanted to kill the boys.
Kamsa had been told by a divine voice from the sky that he would be killed by the eighth son of his sister, Devaki. This eighth son was Krishna.
So the cruel king had sent many demons to find Krishna and finish Him off before he killed Kamsa. But all the demons were killed by Krishna.
Kamsa was very fearful that as Krishna and Balarama were such strong boys, they would grow up to be even stronger and then certainly be able to kill him.
So he thought of this trick of making them participate in a wrestling match, against Chanura and Mushtika, two of his strongest wrestlers who nobody could defeat.
Krishna knew that his was his uncle's plan and accepted the invitation, so that he would get a chance to face the wicked king and finish him off. Then the people of Mathura would be free from Kamsa's torture.