Well my DD is going on 10 and is tall for her age. She tried it and it was uncomfortable. I tried it and honestly I think their are other things that do about the same thing. The Xelerator,which you can make homemade does the same thing except you cannot dial in the resistance.Maybe when they are older it would fit them better. Just my opinion.
My dd has used it for years, we got it just before she turned 10. Its a very convenient indoors workout also for bad weather days. Now in high school with long practices and longer homework, it doesnt get used as regular as it should, but we are still using it.
I have 4 windmills. I loan them out to my more serious pitching students. I think they are well made and a good investment, but they are a tool that fits into a bigger program. Commitment is one of the biggest challenges, I mean how many of us have a stationary bike collecting dust in the garage. The upside is you can probably sell it on ebay for almost what you paid for it. Good Luck
My 12 year old DD got hers last April when she was 11. She uses in 4-5 days a week. It takes less than 5 minutes to do 120 reps of the 6 excercises shown on the CD.
It was designed for equal muscle balance for such a one sided sport (pitching), and to prevent injury. It also helps mantain correct arm and hip mechanics (going from open to closed).
As for increasing speed, it's hard to tell. If your daughter practices hard, correctly, in time she will increase speed, whether or not she has the windmill. Especially when you pick a start date (the day we got the windmill) and a date to be speed checked by her pitching instructor, 8 weeks later. I think the windmill helped, but my DD was really fired up for those 8 weeks - and pushed herself really hard.
IMO - get the windmill. As other posts have stated, if you sell it, you'll get 75% of your money back.