On what date are you intending to travel? A potential date for the reopening of the line to Fort William has very tentatively been put forward for the 11th of July.
As PaulLothian has also described, I would feel lucky to be catching the bus (Though it might be different for a five year old!): The road route between Chrianlarich and Fort William along the A82 offers some of the most stunning scenery and mountain vistas that Scotland has to offer, and in my opinion is far, far and away more interesting than the rail route to Fort William, as good as it is. From the road, you get a far better view of the Horseshoe curve that the train negotiates between Tyndrum and Bridge of Orchy, and the pair of viaducts that span the gap between the shoulders of Beinn Dorain (an easily recognisable conical peak) and Beinn Odhar. The ascent to the Black Mount, where the road breaks away from the course of the railway, takes a twisting course that offers stunning views back towards Tyndrum. Not for nothing is there a well established car park and viewing area here!
The road then skirts the edge of Rannoch Moor, its bleakness obviously apparent and offset by a couple of nice little lochs, before the beginning of the descent into Glencoe. The brooding, craggy mass of Buachaille Etive Mor stands dominant over the entrance to the Glen to the left of the road, and the road twists and turns incessantly down to the valley floor, passing right by the waterfalls at the meeting of three waters, as both the road and the river are channelled through a narrow gap in the gorge (Known as The Study). Along the length of the valley, the present A82 is criss-crossed by the old road, a Drovers Road that was constructed in 1786 and was then improved by Thomas Telford in around 1810. It was replaced by the current alignment in 1930, but remains largely intact, including several of its bridges and culverts and is popular with walkers.
Further down the valley, the jagged profile of the Aonach Eagach ridge, probably the most challenging mountain ridge on the British mainland, rises high above the road, while on the left is the massive hulk of Bidean nam Bian, the tallest Munro in Glencoe. As the road leaves the Glen and curves round to approach the shores of Loch Leven, the location of the digitally super-imposed Hogwarts Castle in the later Harry Potter films is apparent to the right, and the actual location for the set of Hagrids hut, to the right of the Pap of Glencoe.
The crossing of Loch Leven via the Ballachulish Bridge offers excellent views back towards Glencoe village and of the Pap of Glencoe and Bidean to the right, and across Loch Linnhe to the mountains of Ardgour on the left. Even before this, after passing by Glencoe village, look out on the left for an increasingly dilapidated semaphore distant signal planted in the corner of a cemetery, revealing the fact that the current road has been widened along the alignment of the former Caledonian Railway Ballachulish branch. And then its a straight run up the shores of Loch Linnhe to Fort William; look out for the famous paddle steamer Waverley which performs sailings down Loch Linnhe out of Fort William.