Heathcote Reserve in Applecross is great fun – big pirate ship playground, amazing views over the river and plenty of BBQs. Not fenced, so better for kids that you don't need to keep an eye on at all times. From memory there are sand pit play areas (with some small climbing objects) right next to the pirate ship area which would suit very young kids too.
This week I took my 4 year old to a newish play area on Wordsworth Ave in Inglewood. It is near the bowling club and at the end of a huge green, tree lined oval so you can go for a walk later if you like. The play area itself is fully fenced and has all the safety mates and sand that you need to keep them safe. Lots of logs to climb on, swings and those round things that you can spin the kids around on - I have no idea what they are called, but not all the play areas have them. If you have children that are confident you can sit on one of the benches and let them run around. You will still hear them if they need you, because it is not so big an area that you can lose them in it. It had a slide, but not a very tall one, so that was a bit disappointing.
There is a spot called ''The Volcano" near the Belmont Oasis swimming pool which is really good. Its fully fenced and has BBQ's and grassed area for picnics and people are always having kids birthday parties there. My kids love the flying fox and racing up the volcanoe to slide down. If you are there are the right time the volcano even 'smokes'. There's a little kiddies spot and a heap of things for the big kids. Only problem is that you have to trudge around after the kids if they are too young to be left to their own devices - or you could lose them or not be there to help them out if they fall over. The Belmont Forum is just over the way, so you can walk there to get lunch if you need a few extra snacks. The parks there are really green and by the tiem you take a picnic your day will be filled. BTW- its on Abernethy Rd, head down from Grt East Hwy or Lch Hwy depending on where you are coming form.
There is a new adventure playground just finished in Midland, near the river in the area behind or adjacent to the Guildford Grammar school. It has taken forever to be built, I first spotted it when my daughter was 2 and she is now 4! It is still very new though and it is too hot for summer until trees grow in and provide more shade. It's a great space much like Kings Park Lottery west, but there didn't seem to be any BBQs or sitting areas for parents to relax. Maybe they are yet to be built. If you took a day trip, wait till the weather cools a little so it is not too hot. The large Midland shopping centre is just a bit further down the highway, so you can always go there for lunch or a bit of shopping to wrap the day up nicely.