Do children see angels or jinn, and does a baby's laughter while looking at an empty spot in the room mean that he saw an angel?

It is possible and common to see angels and jinn in forms other than their true forms. The Companions saw angels in human forms, and the Prophet, peace and blessings be upon him, saw Gabriel twice in his true form. Some Companions saw angels fighting alongside them in Uhud, and Gabriel appeared to Mary, and the angel appeared to the bald man, the leper, and the blind man.

As for the jinn, it is also possible to see them in forms other than their true forms, such as the story of the devil who came to Abu Hurairah in the form of a human. Seeing angels and jinn in their true forms is possible, but seeing them in the form in which they were created only occurs in exceptional circumstances. And Allah's saying: (Indeed, he sees you, he and his tribe, from where you do not see them) is specific to when he is in the form in which he was created.

As for children specifically seeing jinn and angels, there is no sacred text that proves or denies this, and it is an unseen matter that cannot be definitively affirmed without evidence.