I'm just curious,are there any historical sources that you know of outside the Bible, that would substantiate the claim that the slaughter of the firstborn actually took place?
According to the Catholic Encyclopedia ....
The Greek Liturgy asserts that Herod killed 14,000 boys (ton hagion id chiliadon Nepion), the Syrians speak of 64,000, many medieval authors of 144,000, according to Apoc., xiv, 3. Modern writers reduce the number considerably, since Bethlehem was a rather small town. Knabenbauer brings it down to fifteen or twenty (Evang. S. Matt., I, 104), Bisping to ten or twelve (Evang. S. Matt.), Kellner to about six (Christus and seine Apostel, Freiburg, 1908); cf. "Anzeiger kath. Geistlichk. Deutschl.", 15 Febr., 1909, p. 32. This cruel deed of Herod is not mentioned by the Jewish historian Flavius Josephus, although he relates quite a number of atrocities committed by the king during the last years of his reign. The number of these children was so small that this crime appeared insignificant amongst the other misdeeds of Herod. Macrobius (Saturn., IV, xiv, de Augusto et jocis ejus) relates that when Augustus heard that amongst the boys of two years and under Herod's own son also had been massacred, he said: "It is better to be Herod's hog [ous], than his son [houios]," alluding to the Jewish law of not eating, and consequently not killing, swine. The Middle Ages gave faith to this story; Abelard inserted it in his hymn for the feast of Holy Innocents:
Ad mandatum regis datum generaleBut this "infant" mentioned by Macrobius, is Antipater, the adult son of Herod, who, by command of the dying king was decapitated for having conspired against the life of his father.
nec ipsius infans tutus est a caede.
Ad Augustum hoc delatum risum movit,
et rex mitis de immiti digne lusit:
malum, inquit, est Herodis esse natum.
prodest magis talis regis esse porcum.(Dreves, "Petri Abaelardi Hymnarius Paracletensis", Paris, 1891, pp. 224, 274.)