jetstar is having a sale on fares to tassie atm - there are some excellent deals.
if your DS is still under 2 you wotn have to get him a seat - but is he is older than 2 he will also need his own seat. that makes a diff to price.
imo limit the moving around - ie if you go to tassie stay in 1 or 2 places max - the hasslefactor of too many diff locations...