Excel can't sync 2-ways
As I explained in this similar (but not the same) question, Excel can't sync data two ways. Either you have one master and several slaves (or childs, as you call them) and you can edit the master, but only read the slave. Or you do it the other way around: you can edit the slaves, but you can only read the master.
Another thing Excel can't do (out of the box) is to push data. Excel is designed to sync data "on demand" or "just in time", whenever you open a file or update it's contents. You can't push an edit to another Excel sheet.
If the above are "must have" requirements, Excel isn't the right solution, buy you should look at a solution that is based on a relational database.
I'll briefly explain how to setup the master->slaves and the slaves->master setups, just to get you started.
Master->slaves
Create a table in the master sheet and save it; you could also use a Microsoft Access database for this. Next, create a pivot table in each slave. Set the pivot table filter on the required settings and save the sheet.
Slaves->master
Create a table in each slave. Use Power Pivot to aggregate the different slaves and put them in the master. This solution would also work for a "distributed" approach, in which there is not one master, but all salves connect to each other. That will require two worksheets in each workbook: one with the data (the subset that is managed by that user/department) and one with the pivot report based on Power Pivot, which contains an aggregation of all the data in the different slaves.
Maybe that last mentioned option (the distributed approach) comes closest to your requirements.
Note that if you need to hide a subset of the data on one sheet, you can always put a pivot filter in place and lock the report filter cell.
Best Answer
To extend CharlieRB's answer: set up your pivot table on a new tab. (In the screenshot below I've put the pivot table on the same tab just for clarity.) Be sure to put the "Supplier" field in the "Report Filter" section of the Pivot Table Field List form. I made the Pivot table source cover all of columns A:C on the Master sheet. This allows for automatic inclusion of additional rows upon refresh of the pivot table.
Next, with any cell of the pivot table selected, choose the "Options" menu. On the "PivotTable" submenu of the "Options" menu, click the "Options" dropdown:
Choose "Show Report Filter Pages", and select "Supplier". You'll get a tab for each supplier as you can see in the screen shot above.