We know that Gohan, half saiyan, had a tail.
So why not trunks? I dont think that Vegeta would let anyone remove his his son's tail. The same is with Bulla. Goten too has been shown without a tail. Why dont they have tails?
1. Toriyama answered this question http://www.daizex.com/general/ask/archive/2003_10_12.shtml#6 sorta, going by this and the later explaination, it seems Toriyama was lazy and stopped drawing saiyan tails
2.In universe, Trunkses' tails got cut off by Bullmoose (becuase she knew about the oozaru), i don't know about Goten thought
"Plus it is already said at the Android Saga when Goku ask when is Trunks' tail" nope that was Kurilin, and there was no answer
BTW, Toriyama didn't know how to make female (non armor) oozarus so i assume Bra and Pan oozarus either:
A)Are fully naked
B)Have bra and panties......magic bikini? ^^'