OP - My post was so long I missed a lot.
I take your point - a slice of malt loaf or a yoghurt biscuit isn't a lot. When that's ALL they have. But think of it this way - 2 slices and 2 biscuits and bang, you're already up to 20% of her calories for the day.
Your family meals are totally normal. they're probably also not ideal in this situation. Too high in fat and carbs. Children need both so you shouldn't be cancelling them, but children who like to eat a lot need to fill up on things that are lower calorie, lower carb and lower fat.
eg mince - we now regularly do an asian style mince dish. I use a packet of beef or pork mince which I think is 350 or 400g (bearing in mind that DH has a very very physical job and DS is now 13, 5ft6 and plays sport at a fairly intense level a minimum of 5x per week, depending on matches. There are weeks where he is training/playing matches for 1-2 hours every day). I stir fry this. Then I stir fry a massive pile of vegetables - usually I fill a normal sized chopping board with sliced carrot, onion, broccoli, baby corn, cabbage etc. By volume, raw, it's at least double the amount of meat, maybe 3x. Season and flavour with garlic, ginger, lime, soy sauce etc. And then I add rice noodles or those ready-to-work noodles as per the recommended portion size (which, back in the day, I'd have considered too small). Compare this to when I make bolognaise - the ratio of mince to vegetables, even with the added veggies that I always add (mushrooms, carrots, lentils) is still, by volume, about the same for th meat and the veg. And then there's a huge bowl of spaghetti too....