Squeak is a Smalltalk dialect that was created 20 years ago for educational purposes. But I usually recommend Pharo to everyone, children included.
Here are some great resources:
- a nice, gentle tutorial — Learn Smalltalk with Prof Stef
- the most actively developed Smalltalk — Welcome to Pharo!
- a free book — Updated Pharo by Example
- a Getting Started guide for Pharo — Pharo Quick Start
- a MOOC (massive open online course) — Live Object Programming in Pharo
- some great videos — Smalltalk 4 You
- a book that I like (not based on Pharo) — Smalltalk by Example
- another book that I like (not based on Pharo) — Computer Programming using GNU Smalltalk
- some additional resources — Resources
This book (not free) is my favourite for learning how to use Smalltalk for object-oriented programming: Smalltalk, Objects, and Design, by Chamond Liu.
I myself have published a whole bunch of Smalltalk tutorials. The latest ones are:
From age 10 and up, this should work fine. Younger children may be better off to start with MIT’s Scratch, but this will really depend on the child. Not all children are alike.