So the bit about the war between Jews and the Seleucids is real. It's attested in both the book(s) of Maccabees and in Josephus. I've never seen anyone seriously question its historicity. Granted, the narrative taught in Hebrew school is simplified and distorted, but it's describing a real thing. As an aside, the Greeks didn't sweep in and impose--since Alexander the region had been under Greek rule, and most Jewish sources are actually pro-Alexander. It's also important to note that a number of Jews were Hellenized themselves, and adopted Greek culture and rejected Jewish ritual, a trend which is evident in pretty much everything from Judaism in late antiquity.

However, the bit about the miracle of the oil is probably an addition. It's not mentioned in either the book of Maccabees or in Josephus--the first references are in the Talmud, centuries after the events would've happened. It's also not present in the Hanukkah liturgy, which only references the war. Interestingly, both the liturgy and Josephus explicitly mention kindling the lights of the menorah, but don't mention miraculous oil. Josephus even calls the holiday the "Festival of Lights". Hanukkah is mentioned in passing in the Mishnah (2nd century Jewish text) specifically with lights being kindled, but again there are no references to any miracle.

As to why the story began to circulate, there's no solidly proven reason. Some people have theorized that it was to minimize the role of the revolt (since Jews acquired a distaste for similar revolts after the various wars with Rome, which were quite bloody and didn't end well for the Jewish community), or to minimize the role of the Hasmoneans (since they were sometimes view as usurpers in Jewish texts, and because they eventually did Hellenize themselves, and because their ineffectual rule eventually led to Roman takeover). But there's no solid reason. The bit about references to lighting lights for the holiday before references to the oil makes it a bit interesting, though.

As a partial answer, an annual 8-day festival going back to Judas Maccabeus is certainly described by Josephus, but the earliest explanation of the miracle of the oil comes from the Talmud -- hence, it might not be fair to call it a "kid-friendly fabrication." Maccabees and Josephus make it clear that the celebration is because of the victory involved and thus the restoration of the temple to the people -- the sacrifices and, in the Talmud, the miracle of the oil are simply a part of the celebration of the restoration of the temple.

Now Judas celebrated the festival of the restoration of the sacrifices of the temple for eight days, and omitted no sort of pleasures thereon; but he feasted them upon very rich and splendid sacrifices; and he honored God, and delighted them by hymns and psalms. Nay, they were so very glad at the revival of their customs, when, after a long time of intermission, they unexpectedly had regained the freedom of their worship, that they made it a law for their posterity, that they should keep a festival, on account of the restoration of their temple worship, for eight days. And from that time to this we celebrate this festival, and call it Lights. I suppose the reason was, because this liberty beyond our hopes appeared to us; and that thence was the name given to that festival.

http://www.perseus.tufts.edu/hopper/text?doc=J.+AJ+12.7&fromdoc=Perseus%3Atext%3A1999.01.0146

Until yesterday, my answer would have been quite simple: if you remove the oil miracles, the book of the maccabees is, historically speaking one of the best books of the Bible. In fact it's also one of our best source on the seleucid dynasty and the struggle that began to shook them after the death of Antiochus IV epiphanes (the persecutor of the jews). My presentation would follow the developpement of the maccabean revolt. But I discorvered today that John Ma, professor at Oxford, writing on seleucid history had made a new interpretation on the books on the maccabees. The new reading of the events change what was a persecution in a failed synoecism. A synoecism is the creation of a new city right next to an old one in order to absorb it. In this case the goal would have been to create the greek city of Antiochia (yes, another!) right next to (in?) Jerusalem and granting it greek institutions. The hebrew hellenized elite would be integrated with colonist in the new greek city as citizens but the jews would lose their previous privileges as an autonomous community represented by their great priest. Indeed with the adjunction of a greek city, the autonomy of the jews were compromised and the hellenistic customs such as the seleucid dynastic cult would be practiced in the city. For Ma this integration failed as a petition was sent to the seleucid governor by the great priest Menelaus and his entourage (who are seen as bad in the maccabean texts) to reverse the situation and restore the privileges of the autonomous community. This is of course a much less heroic tale. You can read it here