The new AppCrayon has an ergonomic grip to help kids hold it correctly and it also has an interesting grip sensor which tells then when they are holding the stylus too tightly.
This sounds like a great idea. I help out at my local elementary school and some kids do struggle with this - we get a lot of broken regular crayons!
The AppCrayon Deluxe also has buttons that interact with its iPad app to allow children to change color and erase from the pen itself. (The technology for this is quite neat, it uses an audio signal that humans can't hear but the iPad can detect.)
There's also an ABCs app to help kids learn to form their letters correctly. The AppCrayon Deluxe is a reward for a $30 pledge and there are other options too including a developer kit for those that want to develop apps specifically for the AppCrayon.
Dano Toys Crayon Stylus, 1 ea
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The original yellow AppCrayon is still available to buy and you can get the current AppCrayon app which is free and works with iPhone and iPad devices.
The original pen doesn't have the smart features but is designed to encourage a good grip which should transfer to pencils and pens when kids are writing on paper.
Other Smart Pens for Kids
There are other smart pens on the market or under development to help kids learn to hold a pen and form their letters.
LeapFrog Learn to Write with Mr. Pencil Stylus & Writing App (works with iPhone 4/4s/5, iPod touch 4G & iPad)
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Educational technology provider LeapFrog have a smart stylus pen called Mr. Pencil and there's an accompanying iOS app to help kids to form their letters.
LeapFrog also offer the LeapReader pen which is used on special paper rather than a tablet and help kids learn to read as well as write.
There's also another smart pen running as a Kickstarter campaign which is very interesting. The Lernstift Pen helps children to develop neat writing and also points out spelling errors. It can be used to write on paper and can communicate with apps for additional feedback and interaction.