If you are referring to the PC specs of the average 7th/8th grader (Or the average middle school that I have seen), I’m going to say that Unreal Engine is too high-end. If you are referring to the intelligence level of average 7th/8th graders, I think they could absolutely use/learn it if they wanted to.
To start, I would simply say start with the basics, getting to know the engine. From there, it really depends on where you would like the lessons to go, as there is plenty to be learned.