Child prim Omega by root axis

float angle = 90; //change to whatever angle you need.

integer prim_number = 2; // change to whatever prim number you need to rotate

float spinrate = 1.0; // change spin rate

float gain = 1.0; // change gain

default
{
    state_entry()
    {

        vector RootAxis = llRot2Euler( llGetRootRotation());

        angle = angle * DEG_TO_RAD;

        rotation rot = llAxisAngle2Rot(RootAxis, angle);

        vector euler = llRot2Euler(rot) * RAD_TO_DEG;

        llSetLinkPrimitiveParamsFast(prim_number,[PRIM_OMEGA,euler,spinrate,gain]);
    }
}