Firebase Web get first Child data from autoChildID

My database looks like this: Database image

I am trying to get the first child of unassignedBeaconIDs, read the data beaconID, major, minor and delete it from unassignedBeaconIDs.

Here is my code:

var refBeaconID = firebase.database().ref(pathToBeaconIDs).limitToFirst(1);
        refBeaconID.once('value', function(snap) {
            var firstItem = snap.val(); // first item, in format {"<KEY>": "<VALUE>"}
            console.log("The first item");
            console.log(firstItem.beaconID);
            refBeaconID.child(firstItem.key).removeValue;
            });

The problem is the childAutoID Value. I always get undefined as a result.