That's batty what she told you - what if it happened to be French Onion soup? There are negligible carbs in a whole tureen of that ! (but then you spoil that carb-wise with the half a baguette you serve with it LOL)
I'd do as Redkite says (there are also Apps you can get, or use the Collins Gem CALORIE counter book, which gives Carbs, Protein, Fat, Fibre and Cals per 100g or 100ml) cos it's better to be more precise for a child, especially when you are learning, methinks?
However - Alison is partially right because thicker soup, like this one because some of the starch and the other veg will have leached into the liquid part, is likely to hit the blood a lot quicker than thinner ones eg Spring Veg - so although you might well start with the same amount of total carbs in the veg even though the Spring hasn't any spuds or thickening, because you haven't boiled the veg to mush, you would actually get away without counting all of it. See the logic there?
If you have veg with more fibre - green veg including (small) peas, runner or French beans (the small ones you cook whole) - you can generally ignore the carbs in them - but for eg marrowfat and chick peas or broad and haricot-type beans - you would count the lot. You know even lettuce and cucumber actually contain carbohydrate! - but in negligible amounts so you ignore them.