You should make sure the resistors are high enough, so you do not waste more current then you have to, exact numbers depend on the transistors you want to use / are able to get...
Q2 should be powerfull enough to drive your siren...
if your siren is inductive you should put a reversly biased diode across it to protect the transistor when it gets turned off...
the siren will be on until you reconnect the main battery (or until the secondairy is drained :lol: )