In baseball games, it’s considered a momento to catch a fly ball (a ball which ends up being hit such that it goes into the crowd).

This is making a reference to that but with the Roman coliseum so, instead of a ball, the kid caught a decapitated gladiator’s head.

The joke is basically the inversion of expectations and, possibly, the gross out factor of contrasting the recreation of most sports with the violence of the coliseum.

In baseball the ball is sometimes reaching the tribunes where especially children like to catch the ball and then keep it as a trophy.

This coming parodies it by showing the same thing but with a defeated gladiator’s head.