I have always just bought the ones in the toothbrush section at Woolworths... they are a couple of dollars each.
The first one Aidyn had was a stage 1 Oral B, and didn't seem too big.
The next one we got him to replace that one is a Hi-5 one (don't know the brand) but the bristles on it are longer than the Oral B one, so maybe would be a bit big for Kynan's little mouth?