It depends what you mean by 'low carb'! Cutting down on processed carbs, like pasta/bread and sugar can only be a good thing. But children do need energy and - presumably - yours don't need to lose weight?
I'd focus on making sure that you keep meals based around protein and veg/salad, rather than being pasta/rice-based meals. Look at how often they are eating carbs in things like bread/cereal/crisps as well. And don't fall into the trap of giving them lots of fruit to snack on. Although there's good stuff in fruit, it is (mostly) high in sugar (i.e. carbs). Berries are fine. But if they're using fruit to snack on, switch that to veg - e.g. cucumber, celery, carrot sticks.
Don't buy them fizzy drinks (whether diet or not), if you can possibly help it, and also make sure that they eating a high fat diet. Don't buy any low fat stuff, as it's usually padded out with sugar to make it palatable.
Biscuits, sweets and chocolates are not great, but it's also very hard to not allow children treats like these - and you really don't want to vilify them, otherwise they will only want them more. Just avoid having too much of it in the house, too often.
In general we all eat way too much carbohydrate, so cutting down is always a good thing.