This scene takes place in Chapter 7 of To Kill a Mockingbird. Jem and Scout have just received another interesting gift from the knothole of the tree; this time it was a "tarnished medal." Atticus explained to them that, before they were born, such medallions were given to spelling bee champions in the Maycomb County schools. When Atticus asked where they had gotten it, Scout started to speak, but
Jem camel-kicked me when I tried to say where we had found it.
It is not stated if the children were standing or laying on the floor, but in any case, the "camel-kick" was simply a quick, surprise kick which camels are known to employ.
Harper Lee refers to camels again in Chapter 14 when Dill explains that he had been employed to "wash the camel" at an animal show.
"Camel-kick" appears in chapter seven. In this chapter, Jem and Scout find gifts in the knothole of a tree. They find two images carved in soap. They were carved to look like Jem and Scout. They also find two packs of gum, which they enjoy. Finally, they find a tarnished medal. They show it to Atticus, who says it is from a spelling bee.
At this point Atticus asks Jem if he has showed it to others, because someone must have lost it. As Scout is about to tell Atticus where they found the medal, the text says that Jem camel-kicks Scout.
Jem showed it to Atticus, who said it was a spelling medal, that before we were born the Maycomb County schools had spelling contests and awarded medals to the winners. Atticus said someone must have lost it, and had we asked around? Jem camel-kicked me when I tried to say where we had found it. Jem asked Atticus if he remembered anybody who ever won one, and Atticus said no.
A camel-kick is a powerful and swift kick. If you have ever seen a camel kick a man, you would know the power of it. Jem kicks Scout to keep her quiet so that their secret would remain. If you do a youtube search, you can see camels kick.