I dont understand the safety arguement. in front or behind, the kid is still at risk in an accident.
my thinking is a rear end accident is less likely to be the cyclists fault, (therefore less preventable) than a front or side on accident, which with careful riding, the risk should be reduced.
Also, if you're into something a little more serious, and capable for offroad, checkout the LOCT seats.
http://www.loct.co.uk/
I dont think they have a distributor in Aus. but the shop i work at bought a few from OS- Oxygen Cycles in Mclaren Vale (Adelaide).
Cost is a fair bit more than the weerides though, but a much nicer quality product with greater capabilities imho.
dont mean to spam, but as these aren't available easily in Aus and are a similar product, i thought some folks might be interested.