I bought the Tsunami SP for my 9 year old daughter in the summer and she loves it. It's great to have a kayak that fits a kid well. I teach kayak lessons so I have had a number of kids aged 7 to 12 in this kayak and it worked well for all of them, though I had to adjust the thigh braces, and removed them entirely for one kid. Removing or adjusting the thigh braces takes about 4 minutes with a philips screwdriver.
The kayak is amply stable for my lightweight daughter, and small enough that she is able to handle it well on and off of the water. This summer our family of 4 went canoe/kayak camping, my wife and I paddling a canoe with our younger daughter while the 9 year old paddled her Tsunami SP. We went about 10 km (6 miles) through a series of 3 connected lakes. She paddled the kayak for about the first half and we towed her the rest of the way. The kayak tows very easily. We were camped on a large open lake and we had a short distance to go with larger waves rolling in to our end of the lake. I was concerned, but she LOVED the waves (it wasn't big stuff, but big enough). Once we were into the chain of smaller lakes, the water was much calmer so she sought out the wake of every passing motorboat. Into the next lake, the wind was hitting us hard and so we again towed her the rest of the way home. For that trip, the hatches were packed and she carried at least some of her own gear. The hatches are not large and it is a small boat so don't expect to carry a huge amount of stuff in them, but it is good for them to take responsibility for their own stuff when camping or on day trips.
At 9 years old she is fairly slender but tall. The Tsunami SP affords her lots of room to grow and it should fit her for a few years at least, by which time her sister will be ready to paddle it (that daughter currently paddles a Yost Sea Flea).
The weight of this kayak seems light compared to any "recreational" kayak, or any plastic kayak of any category. (At 12' x 21", it should be light) and I appreciate the weight since I'm the one who has to haul it around.
People usually stick kids into short but very wide kayaks, usually "recreational" kayaks. As far as I'm concerned, this is a mistake. Kids really do benefit from a kayak that fits them. The fact that this kayak is narrow enough that my daughter can engage her knees under the thigh braces means she has much better stability and better control. Proportionally, this is still a pretty wide kayak for her size so it is very stable for her and contributes to great confidence. She has much more fun in this kayak than she does in a kayak that does not fit.
There are very few kayaks out there that actually fit "small persons" and this is one of them. If you have a kid that would fit this kayak, definitely consider it. This is a very real kayak, that they are selling at a price point populated by kayaks of much lesser quality.