Hi hollie, my child is only 3 and so not in school yet, but I thought you might like to hear my thoughts on the subject anyway (since theya re unlikely to change when my child reaches school age)?
dd was a fussy baby with a dairy intolerance and was not putting on any weight. When weaned she detested baby purees, though would (and still will) drink the squeezy ella's kitchen pouches. When shopping for her as a baby, I bought things I thought would appeal to her, whilst still being as close to "home-made" and nutritious as I could buy. She refused to eat almost everything put in front of her if it was "mixed up" ie, shepherds pie, lasagne - yet would happily eat mince, pasta, cheese etc. Obviously when shopping for her then I would again make my choices around what she would eat.
I am of the opinion that you can try and guide your childen to make the right food choices. It is difficult if you have a fussy child, but what you can do as a parent is encourage and offer little in the way of alternatives. dd will now eat most things, but if she refuses she will be offered toast or a banana. There is nothing "nice" for her to eat, nor is she allowed lots of snacks unless thy are fruit or veg (or dairy if I feel she hasnt had much that day).
I don't sweat the crisps/junk food provided she has eaten relatively healthily throughout the day. I make sure she eats at least 5-6 portions of fruit and veg a day and has 3 small portions of dairy. We eat fish at least once a week etc.
Once a week dd has to have a picnic in the car on the way home (us from work, her from grandpaents) so i buy things that she will like and will be excited by. Yes I try to buy organic, but the calorie or otherwise content is not important to me. What is important is that we provide a good role model to our children to eat healthily and to enjoy food, and understand it's value (ie carbohydrates make us able to run faster, brocolli stps us catching colds etc). Everything is fine in modeation, takeaways are not a problem as long as they are not everyday, convenience foods are for us, rather more of an inconvenience, because although we occasionally pick something up fromt he shops, it is actually quicker to make a quick pasta dish yourself than try and heat 3 different meals...
What I think is lacking in today's society is exercise. In the 70's for example, everyone had crisps and mars bars and fizzy drinks as part of their school day. We didn't have the problem with mass obesity then. We are fortunate to live close enough to open moorland and beaches. dd will walk on the moors 5/7 days and on the two days she doesn't she will be with grandparents, normally swimming/at a park/soft play centre etc so getting lots of energy and fresh air.
Exercise and the pre-disposition to enjoy fresh air is something you can teach though. You can make even the most mundane walk into an adventure, a learning game, a hunt for something...there is no excuse for not going out and runnig around. The feel good factor outweighs anything else.
I realise I may have gone off topic a bit, but I wanted you to try and understand why I have made the decisions I did and hold the opinions that I have. I am happy to answer any specific questions, feel fre to PM me.