It's easier to convert them to the same denominator. eg in this case 3 x 7 = 21. So, 1/3 is 7/21 (multiply top and bottom by 7) and 2/7 = 6/21 (multiply top and bottom by 3). 1/3 is therefore bigger than 2/7. It could be difficult to judge that extra 1/21 on a drawing.
But you probably knew this anyway
Robinw, I thought your daughter was too young to worry about the 11+ just yet? Having said that, I remember doing practice papers a while before taking the 11+