As I understand it, the heels press the ground and twist, the toes grip the ground, the head press up and the shoulders press down. All these requirements should be done in a unconsiuous, uncontrolled manner.
I read in this forum that taijiquan imitates the movements and postures of small children, do small children naturally stand like this?

Children (especially those who have yet to attend school) stand perfect because they have yet to "learn" poor posture.
Jake

Rodolfo, I'd never noticed until I heard Tim say it- but it's true- Healthy kids (from the time they learn to walk until they are 4 or 5) generally stand very straight when walking/running or sitting.

They seem to naturally 'expand' their frame internally.

As they age and imitate what they see other's doing they begin to pick up bad habbits.
(slumping, slouching, etc)

Small children stand naturally and correctly, until they are forced to sit for prolonged periods, or are "taught" incorrect body use.

Small children don't "twist" their heels or grip the ground with their toes, neither should an adult standing in Wuji Zhuang.

The lift at the crown of the head and relaxed shoulders are apparent in all children, and anyone with correct posture.

should the heels twist and toes grip in the rest of the form, or should you never do that?

I read that in hsing yi you roll the hips inward, turn the knees in, twist outside with your heels and grip with your toes. Don't they do this in sun style taiji also?

No.