Is it safe for a six year old to take albuterol?

Dose and frequency. The dosing and frequency for her/his age should be followed. Using a spacer device can help to prevent deposition of the inhaled medication in the mouth which can be swallowed.
Yes. Albuterol is even given to infants when needed for bronchospasm (wheezing and coughing). The inhaled dose is <10% of the oral dose so is much less likely to trigger jitteriness or excitement.
Depends. This is a standard medication used from infancy through adulthood. It can be safe and effective if used under proper supervision for the right reasons.