my friends who are also 8-10s usually buy Small Girly shirts from Threadless and its a good fit. form fitting but not tight. i would say get xs tees if you want really tight and fitted, s if you want a decent fit.
for hoodies they tend to buy XS so I assume the sizes for hoodies are a tad larger.
a general guide seems to be
tshirts (girly):
xs = au 6-8
s = au 8-10
m = au 10-12
l = au 12-14
hoodies:
xs = 8-10
s = 10-12
m = 12-14
l = 14-16
this is just a rough guide, but from our recent spree over summer, these size equivalents seem about right. hope that helps!