It's hard to recommend brands at this level. Basically, we usually only buy one or two of these things and never think about them again. Brands change over time too . For example, Repco buy from overseas and rebadge them, some of their products have been really high quality, recently I've only seen cheap stuff. It all depends on what their marketing department is up to.
I'd be wary about buying from K-mart and the like. Yes, they are cheap but they are usually nasty as well. You don't want your kids riding something that breaks. At least in bike shops you know its been assembled properly and you get to see the actual bike you're buying assembled and working before you buy it.
My lad had a good run with Gemini - and he rode all his hard. The lass has one from a second hand shop
Don't worry about brands too much at this level. Just look closely at each bike, fiddle with the bits and make up your own mind about quality.
Richard