I have to agree with AdamS, Elsternwick park has it all! The duck pond is amazing place in Spring with the little ducklings.
In my opinion the best park in the area is Elsternwick Park which is on the NW border of Brighton to Elsternwick. Not only does is have a big play area for the kids, but also has 2 huge ovals for the dogs and a fantastic lake for walking around. Always keeps my kids entertained on the weekend walks.