scar tissue problems

will scar tissue on the lungs caused from pneumonia, (for a toddler), cause problems for them in the future.

[National Jewish Health]

The great majority of pneumonias resolve completely, without any scar formation, especially when there is a quick response to therapy.  However, there are exceptions.  Some very virulent pneumonias can cause:  
• death of lung tissue, called necrosis;
• abscesses; or
• air filled spaces in the lung, called pneumatoceles.
Some pneumonias are accompanied by a collection of fluid or pus in the space between the chest wall and the lung, called the pleura, and these can result in significant scarring.

If the response of your toddler's pneumonia to treatment was rapid resolution, especially if there was x-ray evidence of complete clearing, the likelihood of long term complications, with loss of lung function, will be very small.