Marker animation trigger from parent composition
Totally possible, but there are some problems with your expression.

n = comp("myparentComp").layer("myLayer").marker.key(time); 


In this line n is going to return a marker key object. However the last part of the line uses the current time in the method marker.key(time). This won't work. The marker.key() function expects an int referring to the number of the marker (1-indexed, so the first marker is marker.key(1), because WTF adobe). A marker key has a time property, which is probably where you're getting confused. Also then you're treating n as if it's an integer, and you use a variable marker without declaring it or giving it a value.

If you want the animation to trigger on the first marker, it should look like this:

var marker1time = comp("myparentComp").layer("myLayer").marker.key(1).time;
if(time &lt; marker1time){ 
  t = 0; //hold the animation until the first marker
} else {
    t=time-marker1time; //will start at 0 at the point where time == marker1time
}
thisProperty.valueAtTime(t);


Some basic JS tutorials would probably help. I recommend the w3Schools ones, they're web-design focused, but almost all JS tutes are.