Instead Of Trigger Considerations For Multi-Row Insert

I would like to know how to, if at all possible, to reconstruct the following trigger as to be able to handle multiple row insert when a single insert command is used - because the trigger will only be called once...I'm not familiar and don't know anything about cursors and i've read that its not the best way to go.

TRIGGER ON childtable INSTEAD OF INSERT
AS
BEGIN
DECLARE @customkey char(16);
DECLARE @nextchild int;
DECLARE @parent int;
DECLARE @date datetime;

SET @date = getdate();

SELECT @parent = parenttable FROM inserted;

SELECT @nextchild=count(*)+1 FROM childtable WHERE parenttable = @parent;

IF (@nextchild >= 9998) return;

SET @customkey = €˜type€™+ convert(char(4),year(@date)) + convert(char(2),month(@date)) + convert(char(2),day(@date))+convert(char(4),@nextchild + 1);

INSERT INTO childtable (customkey,parent) VALUES (@customkey,@parent);
END