OK.... I'm going to start by pointing out why this should NEVER happen. I was a Cub Scout leader for almost 10 years, so I see protocal that was not followed. Cub Scouts are not allowed to camp with just 2 adults. There is the rule that it is manditory to have a minimum of 2 leader and youth protection trained adults at any and all functions (2 deep leadership). When Cubs go camping, it is manditory for there to be a parent or guardian (or person who the parent/guardian signs a waiver to be there in the place of that parent/guardian) for each Cub. This is because Cubs are in the age group where they still act before thinking, and it's important to have those extra adults around to supervise. Therefore, the scenerio that you put here should never be for Cub Scouts, and the leader will be removed by the District Executive after the incident and not be allowed to reapply as a leader.
Looking at the scenero and what we've taught the Cub Scouts and Boy Scouts that I've worked with here's my comments on what to take.
The best things to take in are going to be a tarp (there should be at least one under each tent so take your pick), a sleeping bag, the jump kit, rope, twine if you have it, a big towel or sheet, and a team of the boys that are picked for endurance and physical shape. Make sure 2 of your boys are tagged to mark the trail so SAR can follow the trail into the area (and you can follow it out). The rest of your team is responsible for helping look for 2 thick branches that are as close as possible in length to the injured boy's height and some smaller reasonablly straight sticks for splints.
I agree with the earlier post that you're likely to need SAR for a technical rescue depending on how deep the ravine is so be prepared to spend time talking to the kiddo while SAR gets there. If you are able to get to the kiddo, you can use the towel for a c-collar if there isn't one in your jump kit. Splint what's needed with sticks and the towel or triangle bandage. Make a litter with the big sticks, tarp and twine or rope. Get the child into the sleeping bag and then zip it closed around him. Pull the sleeping bag onto the litter. Then, you work as a team with the other boys to get the injuried boy from the scene to the camp. Try to have a couple of extra boys to swap out those who get tired or stop and rest if you need and keep checking the boy as you go.