From experience there will be a 3% chance of wet weather. Even less so that it will be happening during your party.

In the past 5 years there has been only a few occasions where it was simply too wet to run the activities outdoors. In all but one occasion the forecast gave everyone enough time to organise alternative arrangements. 

I check the forecast daily on the BOM website. It is the most reliable and accurate  If 3-4 days before your party it forecasts “rain” I will contact you to give you a "heads up" and see if you want to postpone, cancel or find a suitable indoor location (see "Plan B" below)

If you cancel there is no penalty fee. A final decision can be left till a few hours before the party if need be.

Do not be alarmed if you see a forecast stating “possible shower”, “few showers”, “showers” it is usually not a problem. The forecast is for the whole of the Greater Brisbane Area and the shower may end up happening in only one suburb for a minute. Basically someone has cursed you if there is even a passing shower during your party

Totally ignore commercial radio forecasts as they are hopelessly inaccurate. They consistently forecast “a wet weekend ahead” when the truth is there may only be a possible shower - somewhere. It is far easier for them to say "rain" than give an accurate forecast. Below is the liink to the Bureau Of Meterology (BOM) website

"Plan B" - Activities can be run indoors at the following venue types: school hall or an undercover area; indoor cricket centre; sports centre; PCYC; YMCA; community hall; church hall; scout hall or girl guides hall.The bigger the kids or larger the group the more space is needed. A dozen or so 6yo's in a scout hall would work but obviously not with 10yo's or say 20+ x 6yo's. For that age or number a basketball size court area is more suitable.  No matter what their age the more room they have the more fun it is