I just flew to Melbourne and didn't bother to bring a birth certificate to proove Ella is under 2. I went with Virgin Blue and wasn't asked at all.....but then again Ella is only 18months and looks like she's about 10 months - lol! It does seem silly to have to pay for a whole seat if she's likely to sit with you or DH anyway - and she is small. I'd see what they say.