Gosh, so sorry to hear this and it makes me sad because I have 2 teen boys, and they eat loads.
All I can suggest is that you try and source a cheaper way to shop for better ingredients. Also, don't throw anything away. Even though I am not broke, I use everything.
Not sure if this helps but these are some of the cost savings/ lower waste things I have done.
I NEVER buy sliced meat. I always buy the largest chicken I can find (at a good price so Aldi/ Lidl) and I cook it, and strip it bare for meals, and sandwiches. I then boil the carcass in water with a carrot and onion for stock.
Some fruit and veg is cheap, although not sure about France. Carrots are cheap, bananas are and they are a great piece of fruit. If you are making a meal, don't plate up, but let them take what they want. If there is anything left freeze it. I take all left over veg (plus the carrot and onion from the stock) and make soup. I'll bung in bits off the chicken I stripped. This with bread, is another meal.
I think you need to swap anything like a cereal bar out for fruit and nuts. If you want to improve their nutrition, you need to make sure you are spending your money wisely, and don't buy processed bars like this, as they are expensive and generally nutritionally redundant.
I think we Brits bin stuff unnecessarily. If I am not using something, I freeze it. Someone from overseas once couldn't believe I cut the stem off broccoli and binned it. She cleaned it and cut it into long small strips and then sautéed them. Apparently they are the most nutritional bit of the broccoli.
You can learn about nutrition. Looks around the shops, markets, and other places and see how much the meat, fruit and veg is. Make a chart, compare them. Look to see what fruit and veg is ££ and what is more bang for your buck. You can come back here and ask and we will help you.
As said though, you need to be more savvy and you can learn as you go. That's what we all have done. No one gets nutrition ed at school.