I think the Honda 300ex has smaller ergonomics for your daughter (use the screw to limit throttle as suggested) and that quad will be great for her at any age. The 9 year old boy, depends on his size and experience. The Yamaha 200 blaster is a great quad on a smaller frame, it is probably way too fast for a newbie out of the box but then again, use the throttle screw. The great thing about the blaster is that you can add hop ups when he get older and more experienced to keep him entertained.

Ultimately, they have to comfortably fit the controls and be experienced enough to use the power and know how to ride.

I have two 10 year olds that have been riding LT80's since they were 5. They still fit comfortably on the machines but dont have the power to match their abilities. They are experimenting with my honda 400ex. I am questioning myself on whether to buy the yamaha blaster or go with throttling back a 400cc class quad. I had a 400 lb. quad land on me and it messed me up. I cant imagine what a 400 lb machine would do to a 100 lb. kid, even one with 5 years riding experience. Good luck in your decision.