I spent some time looking around Kathy's website. I've used her site before as she has a ton of great resources.
A new web tool I came across was ABCya animate. It's an easy to use animation tool with a tutorial to get you started. I liked how kid friendly it was. The downside is that it only saves as a .gif file and is limited to 100 slides. However, this is a great introduction for kids about animations. From there, kids could move into using Keynote as a similar tool to create animations.
Other animations tools my kids love are PowToon as well as ToonTastic and PuppetPals apps on the iPad.
I'm curious to know what other animation tools teachers are using with students.