Checkbox with additional text won't render in datagrid

The checkbox feature in datagrids using brackets "[]" doesn't seem to work if there's any other text in the cell, for example "[] Foo" will render as open and closing brackets with the text "Foo" on the right.

The reason I need this is that I'll have more than one checkbox in a cell because it's not possible (as far as I know) to create more advanced tables with Balsamiq Mockups. A cell that has three rows of headers and 8 columns for example (level 1 has one header, level 2 has 4, and level 3 has 8).

I created the following datagrid using three datagrids and some fine-tuning of text in lower-layer cells to align all the columns.