There are so many good places in Perth to take kids during the holidays. A lot of shopping centres have free school holiday activities, as do most public libraries. If you can get into the city a few times, there are lots of free and affordable holiday activities at the Perth Cultural Centre. For example, The Place at the State Library is a purpose-built, indoor play for kids (which often has holiday programs for different age groups), the Discovery Centre at the WA Museum has lots of fun educational activities and the Art Gallery of WA always has something for children. I think that PICA (Perth INstitute of Contemporary Art) also has holiday activities but I don't think they're free.
YMCA HQ in Leederville offeres Saturday morning skateboarding lessons - every weekend! They also do private lessons and group bookings - not a bad idea for the next birthday party... Why not get the kids outside for some physical activity, and watch them master a new skill!