Police are appealing for witnesses following the attack in Derwendeg Avenue in Cefn Hengoed.
The assault happened at around 11.45am on Saturday, April 15, and left the boy with a facial injury.
Gwent Police said a boy and a girl, aged between 12 and 14-years-old, had approached their victim asking questions, before attacking him with what is believed to be a rock.
The girl is described as wearing a grey, hooded-top, blue tracksuit bottoms with red/brown shoulder-length hair.
The boy is described as wearing a blue hooded top, grey tracksuit bottoms with short blond/brown hair.
Anyone with information can call Gwent Police on 101 quoting log number 169 of 15/04/2017.