Not sure where Mintox thinks Bayswater is but it is right next door to Morley. The Starmites in Bayswater is about 2 mins from the Galleria, so if you are in Dianella with access to walter Rd it would take about 5 mins to get there. Loftus Centre in Leederville is also easy to get to straight down Fitzgerald and then head to Leederville. Logically any of them could be okay depending on your location exact location in Dianella.
Your best bet is to check for any in Morley. I do know of at least one Gymnastics Centre located in Malaga, the industrial centre located about 10 minutes north of Dianella. Logistically, this would be a better choice than Leedervile or Bayswater
You could go to Star Mites in either Leederville or Bayswater. They are the only one I know of that is even near the inner - 10-12km -city areas. They are on the web, which will give you class listings and times. They take little kiddies from 2 or 2.5 in toddler classes as well.