Hi CatMeow. Just to share with you my experience.
I wanted to book Yio Chu Kang Grassroot Club too but when I wanted to confirm, they are already fully booked and I wanted to book 3 months in advance. That place is very "hot" because it is very convenient.
Then I went around Singapore, to those play houses where they can host birthday parties. SUPER EXPENSIVE. And I realised those are catered mainly for children (not infants or toddlers).
Atlantis City @ Delta : This playhouse is very good. It is quite small and ulu but this is the most flexible venue. As in, you can order your own caterer at no extra charge. To book the whole venue, for 3hours, is about $350. They close the whole place for you only.
The Play House : Very nice venue but unless you have A LOT of cash to spare, otherwise dont't have to think about going there. The place is only accessible if you have vehicles. For those guests who take buses, have to climb up the hill which I think, will make noise when they arrive at the party.
Fidgets : Didn't go down to take a look as their rates are not cheap also.
Polliwogs : The price quoted is only for children food. Have to order additional catering for adults which I believe will add up to 1K so I drop this place.
Hotel Function Rooms are usually around 4digits after I checked, even those ulu hotels also. So no point...
At the end, I went and booked a multi-purpose room at my neighborhood CC. Cheaper and better. Just get your own caterer and your own decorations. Rent some party toys and the whole place is good for adults and kids.
Hope this will help...