Just to add a personal expirience about children, programming, planning etc.

Because of my heavy work ( mostly at home ) my son got in touch with
computers early.
If he was 4 he got his own PC - left from a visit of one of our foreign
researchers.

He is attracted by LEGO technics/mindstorms ( to create all this robots
requires a lot of planning ).
He likes the "incredible machine" very much and creates now his own puzzles
with this program ( now he is 7 )

Also he creates quite functional "programs" in on old PARTS Workbench (
ObjectShare Visual-Smalltalk ) using some PARTS which I made for him to
interface Lego cybermaster.
Visual Programming enables to learn incrementally - it is very easy to
drag/drop a Window in the Workbench, place a PushButton on it, connect it to
the Speaker - which was my son's  "first program".

He also likes the "Physicus" Multimedia Adventure learning game, which
requires a lot of planning.

For Squeak I believe it would be very nice to have such "games" like
Incredible-Machine, Physikus and interface to Lego to attract childs.

First childs needs to get attracted, then software must enable incremental
learning and it should be possible to make more complex things to stay
attracted. This will lead automatically to the discovery of "programming".

Frank