For younger kids, especially those just learning, Kensington's JJ Holland Park is great. It has a compacted granite jogging track the whole way around. It has many different slopes of varying degrees and surfaces - really every possible option for those first tentative attempts. Toilets. BBQs. Tables.
Moving on, there's the BMX track in the corner so kids can try their skills. And later again, the skate park gets a lot of BMX action. (The 'BMX' track is due to be revamped soon, with kickers of varying heights/difficulty, as well as a pump track).
Not sure if it's worth a trip to, however, if there's something closer by.