Model rockets should never be built with materials such as PVC tubing and rocket engines should not be attached to anything that is not designed to be powered by them. This is extremely dangerous. The materials should be something that will disintegrate if it impacts something. Paper, balsawood or thin plastics.
I also don't think attaching motors to your RC truck would do much if anything, but be dangerous. If your RC truck weighs anywhere near what my sons does the engine might not even move it. Again, it is very dangerous and you shouldn't attempt it.
I know these are not the answers you are hoping for, but there is a reason they require 'adult supervision' when launching rockets. They can be dangerous if they are not used as they are designed. Model rocket kits have been design to be safe. Checking the CB/CP and many other things. Putting things on any rocket that is not design for it could cause serious injures.
Rocket bodies are very difficult to make. I have only heard of one person successfully making one. What he had done was to grease a stick that he wanted as an inside diameter of the tube. Then wrapped that stick with brown craft paper soaked in white glue. Let it set for a few days and slid the stick out. Nose cones are hard if you don't have a lathe. Trying to get them symmetrical would be difficult. The nose cone and fins are usually made out of balsa wood or plastic. Larger rockets will use plywood for the fins. Body tubes are always some form of a paper/cardboard tube.
Depending on how you want to run your launches. If you are only going to need one or two launch pads and launch controls, Wal-Mart is your best bet for 'Starter' kits ($20). They will include everything you need for your first launch but batteries. Usually the rockets in the starter kits are pre built or need very little assembly. If you want the campers to build their rockets, look for skill level one kits.
The 'Alpha' has been around for 30+ years is simple to build and is very stable. It used to be the rocket everyone started with. If you are in need for a lot of kits try EBay. EBay seems to be the only place you can find 'educator' sets.
They would include a starter kit and 12 skills level one rocket. Usually 'Yankees', another good starter rocket and should be about $4 a rocket. The last thought that comes to mind is your local hobby store. Tell them what you are doing and most 'mom and pop' hobby stores should give you a pretty good price. Este's rockets I think are the best kits for beginners. They seem to have a better fit and finish to them. Remember the skill level one rockets are going to need glue (both white and model glue), paint, and maybe sand paper.
I would think it would be reliable. Just as any staged rocket, the distance from one engine to the next would be the biggest variable that would affect 'light off'. But, I do not have a lot of experience with staged rockets. My launch sites are not big enough for safe recovery. My only experiences with staged rockets are with C engines. I have never had a failure with one stage lighting the next.
These have been Este's kits; I have not designed one myself. On the other hand, Estes does not make and E' engine booster, that I know of. There might be a reason for that. Might be an altitude issue, maybe a light off issue or a weight to thrust issue?
Model rockets have an ejection charge that will push the nose cone and parachute or streamer out. In a tumble recovery rocket, the ejection charge will push the engine casing out the back of the rocket making the rocket unstable and it will tumble back to the ground. This ejection charge is timed from the end of the propulsion phase, which is what shoots the rocket upwards.
If you look at the numbers on a model rocket engine like an Estes A8-3. The A8 tells you how much impulse power it has and how long it will burn. The '3' tells you that 3 seconds after it finishes burning the propulsion phase, it will pop the ejection charge. The reason it doesn't pop right after it's done burning the propulsion phase is the rocket can be traveling at 300mph and the little plastic parachute would rip off. Also the rocket continues to fly higher because of momentum.