I'm sure there are kids out there with worse diets! I managed on £25 a week for a few months due to losing a job and being beyond skint and it was repetitive and dull, but we managed because we had no choice. I'm still not exactly flush, but I average about £40 a week now for me and ds(8) but this includes household stuff, toiletries etc. I've just spent £57 at Aldi, but we were running low on stuff like washing powder and tea bags, so it was a bit more this week.
My ds is picky and will only eat a select few pieces of fresh fruit and veg (carrots, cucumber, apples, strawberries, grapes, pears and mushrooms if very tiny!) but he likes tinned mandarin oranges, peaches and pineapple too, and I can get away with puréeing tinned tomatoes with onions, garlic etc to serve with pasta, or as a base for a pitta bread pizza. He also loves raisins, sultanas and dried apricots, so he has these for snacks and on his weetabix. He doesn't drink milk or eat bananas, but he loves a couple of bananas in half a pint of milk whizzed through the blender, and prefers his carrots raw unless cooked in a cottage pie or something. He's a very little eater, and I'm never convinced he's getting enough vitamins so I give him a supplement just to be safe.
I agree with PP's that you need to look at their calcium intake and maybe look at introducing a vitamin supplement alongside what they eat.