Because even if the abductor had covered their mouth, still the muffled screams would have come alerting people in the silent background, at least the kids would have tried to kick and punch even if it's just the ground or air in front of them, which could have left a mark or traces, giving the people searching them some hint at the very least. Due to the nearness of District Five to the desert, this place hasn't seen the face of rain for ages now. So, the traces wouldn't have disappeared and even if we are taking the possibility of this being the doing of a group.