All things considered

Having carefully planned the group formations as you did, there were still plenty of things that simple marching formations couldn't solve. One of those things is the logistical nightmare of organizing a mass exodus when your kind have never even gathered in these numbers before, let alone MOVED in them. All of the paths that the Ardri know best and use most frequently are long and winding, intended to find good game and avoid dangers. These paths were often quite narrow as they never had to accommodate more than a few dozen warriors. Such paths would be of little use in this journey. Though the great river was a place of great danger and mystery it was the shortest route to Gidul, not to mention it would almost certainly be safer in the end than the alternative routes.

Going through the mountains would be the most foolhardy of choices by far. There were many passes and gorges much too narrow to allow many people to pass at once and some of them only had room for one at a time, your numbers wouldn't allow for that. That was also not to mention the predators of the mountain, the Drakes would have little difficulty in picking your numbers apart all day, every day, and the cave spider packs would surely claim a few. Those two predators alone would be both clever enough and numerous enough to be a significant threat to almost anyone here, especially the children. Not to mention that the mountain paths would take you at least a few days more than the valley paths would.

The foothills would not be likely to have any drakes, however, creatures like the Moruga and Bristlebear were fairly common to those hills in addition to the Greater Elk and a uncomfortably large population of Nadal. These paths were very frequently used by the Ardri however and therefore a Raghik attack could be more than likely. Unless your group was the only group that was attacked, a Raghik would be capable of ending a great many lives. The possibility of it even wiping out an entire group was not entirely out of the question.

The river path however was fast. The great river runs southeast toward Gidul's Sanctuary and then meanders east to the sea. The River changes direction about a half-days hike from Gidul. This path should only take about a week. Plenty of dangers lie along the river of course but these are typically easy to spot and avoid. Best of all, the terrain was wide which would allow much faster passage. With nothing but pure forest and riverbank, plus a few backwaters and creeks, the choice seemed clear to you indeed.