If your kid is under 65 lbs. the Fun 1 can't be beat. It is narrow and shallow enough for the smaller kids. My daughter started in her's at age 4 and is still loving it at age 9 and 65lbs.
The big issue with kids kayaks is the width and the depth. Both will make rolling really hard even with a ton of foam.
She also has a Sidekick and it is a very confidence inspiring design and her go to boat for pushing her limits in larger water and river runs. BUT, she is sitting on two sweet cheeks and has a ton of foam in the hips. Her Fun one is the boat she has the most confidence in rolling. I think at age 10 this summer it will really start to fit here well.
The Fun 1 has been the go to boat for working on skills and getting her roll down. She can now roll both ways with a paddle and just with her hands hands. She is starting to work on freestyle moves more and we will tradition into the Shooting Star over the summer as she is growing fast now.
If you are really serious about paddling with your kid then you should also consider a Dynamic Duo. It will allow you to take your child on some harder/longer runs and give he/she the feel with out being on his own. We have done many class III+ rivers and it has taught many skills that transfer into my daughter being in her own boat. Think of it like training wheels on a bike. We can roll the Duo together or on my own if she just tucks. It's a blast to surf and we even like to boot and splat rocks as a couple.
Jackson Kayak has put more R&D $ and time into kids boats than any other manufacturer. The only company offering a kids river runner (fun 1,1.5), Play (Shooting Star), and creeker (Sidekick & Little Hero) in multiple sizes, They even had a kids rec. boat for lakes. Talking success look at what that has gotten the Jackson family. Emily and Dane are both world champions and this comes from having the tools to excel at a young age. Many of the other brands/designs work well for older kids but they aren't really narrow and shallow enough for kids 30-60 lbs. Once you hit the 95+ lbs. many of the small adult boats will work great.
I salute anyone getting kids started in paddling. It is on of the most rewarding things you can do as a paddler. Also check out Colorado Whitewater Kids Team and The Lyons Kids Kayak Club if you are in the Denver area and looking for a peer/ instructional group for kids paddling.