There is a little play ground for younger children in St Marys Shopping Village, while it is little it is definitely indoors, air conditioned, and near a toilet. It is free, and whenever I go to the Target there my kids beg me to let them play even though honestly they are too old for this play area. Clearly it does the trick. The Centre has a few cheap food options to get a treat for a child too (like the donut place or McDonalds) and there is a Woolworths in the Centre.
Parking can get a little rough on weekends or peak shopping times, and I imagine that near Christmas the fact that it is so close to Target will make it very popular. This is a good option for a poor weather day, that is free.