I have a very large and well made messenger bag. It does the trick. Little sweatier, but no extra weight as with a rack. Besides, I have to put the kid on the kiddie seat/rack two days a week, so panniers wouldn't cut it.
Panniers also add wind resistance. My bag is negligable.
__________________
The bicycle, the bicycle surely, should always be the vehicle of novelists and poets. Christopher Morley