Actually they didn't have boys clothes, Dallas alters the spare clothes they have to fit. At night round the campfire you see Dallas altering the jacket and the stitching shows its been cut down. Same with trousers.
When McCrea brings in Dean Stockwell, several on the crew do not want him because picking up strangers on a cattle drive are bad luck. YET, when Stockwell tears his pants and needs a new set of clothes there just happens to be an outfit that fits a 15 year old boy when there are no boys on the cattle crew.