I've only been here once with my wife & 2 Year Old but we thought it was really nice. I haven't been in the mansion but the grounds and garden were beautiful and enjoyable to explore. Its amazing to consider someone lived here and how much land there is in Elsternwick. Must be worth a lot! I will visit again!
It opens at 10am (wish earlier), closes at 4pm and it cost $30 for 2 adults (2YO free I think). This is pretty hefty price given Botanical gardens are free but it is beautiful and seeing inside the actual mansion would add more value I believe. We couldn't buy a 'garden only' ticket that day due to some band playing later in the gardens. I think its $10 each adult for garden only normally.
The gardens really are beautiful and big. We stayed 60-90 min and didn't see it all (2yo's walk slow though). There are lakes, water lily ponds, bridges (fake wood, which is bizarre), waterfalls, open grass areas and my favourite part is the lookout which is fun to get to and gives nice views of the grounds. No one was around when we went at 10am. As we were leaving at around 11-11:30am, we saw heaps of ppl, so go at opening to get some peace. There is also a kids garden in development, but there was only a turtle shell sculpture when we went. Worth a look though (on left as you enter).
There is a café and nursery, but we didn't really look at either as we knew we didn't have much time.
The mansion itself looked large and old, but we didn't have time to go on one of the tours that is free with entry. It would have been interesting but I assume boring for a 2yo so we prioritised the garden.
I'll go again and take a picnic lunch and spend more time enjoying the beautiful garden. Maybe brave a tour of the house with my 2yo.