So my take on this is that it is really hard to answer without knowing the reason for the question. Let me give you an example what I mean. When my son, AJ, was about halfway through his driving lessons an opportunity presented itself for me to get an older Fiser car which was too large and too tall for AJ to really race in. However I bought it because $1200 for a complete race ready car with both a 120 and a 160 and a big box of spare parts was too good to pass up. We put AJ in it for about 3 hours of practice a week for a couple months and he raced his first 4 rookie races in that car before we bought the proper sized car for him (here is a picture of the huge car:
http://www.freitasracing.com/photogallery/22209/Race1a.JPG).
We bought the car for $1200, sold the 160 for $300 and then sold the car for $1500 six months later (we did rebuild the shocks, made new bumpers and nerfs and basically went through the whole car to make sure everything was in good shape). In that time AJ got a bunch of seat time and my brother and I learned all about how to work on these cars that we had no knowledge of prior. For those reasons I think we made a great decision putting AJ in that car.
At the same time I will say that AJ was 5, so I had time on my side. If your child is 8 they really can't afford to lose too much time due to having the wrong car because once they hit 9 and have to jump into the Sr Honda from the rookie class it is very tough on them.
I hope that helps.