My dd used to have a piece of chocolate cake and a piece of bread and butter for her lunch. For years. To be honest she didn't eat much better out of school either! The family ate 3 good meals a day - the rest of us (and the other children) ate oily fish, veg, meats, fruit etc etc. But if you saw her at school you could assume she was fed poorly. She was the equivalent of this child but definitely day in, day out.
Funnily enough I am a pretty good parent, fairly knowledgeable in nutrition and we are comfortably off etc. Certainly no depression, lack of money, poor food choices etc here.
DD was a fussy old fish - it was more important to get food into her than nothing. She had had referrals to dieticians and we had sought private help. Nothing did help - she just seemed intolerant to most foods (or the taste of them). She had prescription vitamin supplements at night and fruit juice etc - nutrition where we could get it into her and she did ok, growing well, healthy etc.
She is now a happy healthy teenager who eats like a horse, in the past couple of years has started being able to eat almost all foods - full three course meals of salads, veg, fish, meat etc;, plays loads of sport etc etc. She has a really healthy attitude to food, is very slim but not thin, would choose to make sure she has at least 5 fruit or veg a day but has chocolate etc when she feel she needs it. At the moment I am impressed by her attitude to food. This is the plain bread, plain pasta, chocolate cake eating toddler, young child that we had for years.
Its hard not to judge but unless there are known other problems, it is best not to!!!! The nursery will be-able to tell if the child is malnourished or hungry - and shock, horror - the parents may already have spoken to the nursery about the situation. A parents seeing a snapshot of the situation cannot really see the true picture. If the child was not in school or nursery, and out of the 'public eye' it would be more concerning.