Depends on what you are wanting to pay.
Sarti's, in Bath Street and Wellington Street (they're just around the corner form each other and are in fact linked inside) is a good Italian place, which always welcomes kids, but can get crowded at lunchtimes (no booking) - but you can book in the evening. It does the whole range of Italian food - pasta, pizza, risotto, bruschetta, salads, meat dishes... (.... my stomach is rumbling! ) Sarti's also do a great breakfast - the best value in Glasgow: a glass of freshly squeezed orange juice, a couple of slices of Italian bread toast (or a large croissant) and a good coffee - all for £2.50
Buzzy Wares, downstairs in Princes Square (off Buchanan Street), which does a range of Amercian style burgers, nachos and greek lamb dishes, is also good with kids. If they're old enough and you can get a table on the "outside", the kids loves to run around and play on the mosaic (the central area of the atrium), and you can watch them. (Lots of parents just sit on the steps and have a relax while the kids run around).
The cafe on the top floor of John Lewis in Buchanan Galleries also seems to be quite friendly - they do a nice wee children's box.
There is also the usual range of Pizza Express, TGI Fridays....