You can lookup the stats per age group on the MAA site, but 8 is very close to the average score for 7th grade. So your 12yo is basically on track.
I think it would send the wrong message to not take the test. Hopefully, it will be a fun, motivating experience and you'll have a baseline to compare for growth if you retake it in 8th grade.
Also in 2 weeks I don't think you can radically change your score after becoming familiar with the format, but over a year if you're interested by looking at variety of problems you can definitely grow.
