The Mums are going to enjoy it if the coffee and snacks available are good quality!! There are lots of playroom cafes in Australia and UK, but often they skimp on the quality of the food and beverages which is disappointing when you want to relax and enjoy yourself.
I don't mean you have to have the most expensive espresso, but at least make a bit of an effort with the quality rather than writing it off as the cheapest part of the budget.
Ball pits are always very popular, jumping castles etc likewise (though a little risky I guess). My nearly 3 year old loves any big climbing frames, like the huge ones you get at some play centres, with netting for walls, and lots and lots of sections to explore, with slides coming down etc.
We visited this place in the UK, which is what I'm talking about (look at the pictures down the side):
http://www.thegiantsden.co.uk/parties.html
I agree that you need excellent, enthusiastic and qualified supervisors (not just staff from other roles in the hotel taking turns looking after the kids' room), don't assume big kids won't want to play with the little kid stuff! You always get ratbag 9 year old boys wanting to jump on the baby equipment and usually in these kinds of places, parents don't supervise their kids very well.
If the supervising staff don't have a genuine interest in children, it will be very obvious, they'll get bored, accidents will happen, nobody will have fun.