I just bought the Alesis DM6 for my 6 year old son. It is very nice that he can practice when his little brother is sleeping or whenever.
But, I know it would not be a waste of money because if he doesnt use it I will.
There are some cool tutorials for converting an acoustic into an electric kit as well which looks like a bit of fun, check out:
http://www.edrums.info/
They also have pads at on the acoustic kits at the music school which quietens things down alot, maybe something to think about too.
My sons drumming teacher also says the benifits of having an electric kit at this early stage outweigh an acoustic.