Toddler got constipated and started taking miralax, (polyethylene glycol) but now if she stops taking it she gets constipated right away. is it safe for her keep taking it?

It's safe: If this is just a one time thing for a few days, usually just some extra fluids and/or juice. If it is a long term problem, then some diet changes with less milk products and more fiber needed. In addition, a product called MiraLAX (generic peg or polyethylene glycol) used on a routine basis is helpful. It is very safe and not an addicting laxative - just helps attract more water into the stool.