I'm a high school student who has competed for two years in VEX competition and am now building an OpenROV (from scratch, but I'm confident my experience will apply to the kit). I'll give you my perspective from having done both.
Building isn't that much more labor intensive than VEX because the epoxy is much easier to deal with than all the tiny vex screws. That being said, it will require some extra care to make sure nothing goes wrong, as here any failure could wreck $150 of electronics. This might, however, be a good way to have the kids figure out how to test the electronics and battery tubes before they put the electronics in.
Programming could be another story. If you are just using the standard design, you don't really need to worry about anything more than loading the images to the SD card. I don't know which language you use for the VEX programming, but arduino code at least isn't too big of a step up from the RobotC or the more advanced versions of easyC.
I guess what it really comes down to is that everything is certainly doable as long as adults are prepared to jump in when necessary. I think this is a great way to get kids into the more real-world side of robotics (which is basically how I pitched the project to my computer science teacher). Additionally, much of the initial difficulty is building it; once you have one running, programming or adding various enhancements (depending on their complexity) is fairly easy.