Fighting Nature

We live in sub-tropical Queensland 150km north west of Brisbane.  We really have only 2 seasons, a hot wet summer and a long dry winter (well this is what we should get) with a short spell of warm settled weather either side.  At the moment in what most people call spring our day time temperatures are around 28-30 degrees celsius and our night temperatures are around 8 degrees.
This far north we can grow plants such as bananas, paw paws, and mangos.

Bananas picked at the start of August

But because we live away from the coast we also get frosts.  Now when I tell most people that we get frosts out here they think, because we are in the subtropics, that I mean the odd patch of frost covered grass in a shady hollow.
But what I am talking about is a completely white backyard with ice crystals on the plants.  These next 4 photos were taken at the end of July.

The wheel barrow track left in the frost after feed hay to the cows.

Ice crystals on the brassicas

Frosty brassicas

Jessie is not worried about the frost as long as she
can play with her road cone toy 

So even though we can grow these tropical plants there is a good chance they will take a battering in winter.
There are of course things we can do to protect from frost damage for the short period of time that it is an issue, and at this stage the mango is only young so will hopefully be a bit tougher when it is a bit older.  The bananas and paw paws do take a bit of a battering from the frost but are quick to recover and are a quick plant to re-establish.
So with these cool temperatures in mind I raised a range of brassica seedlings so that we would be able to grow winter vegetables.  And grow they did.  I planted them into my experimental garden bed  back at the end of May and they are strong and healthy.

What they are not doing however is setting fruit, all I have are leaves.  Healthy leaves.  And now I am worried that it is too warm and nothing will come of all these healthy plants.  The only one that has done ok are the mini cabbages.  Because they are small they formed solid hearts more quickly.
So am I fighting nature?  Do I give up on trying to grow cool climate brassicas?

If all else fails does anyone have any recipes to use the leaves?
Or did I just grow a crop of chicken food?