You will save by shopping in Aldi/Lidl, but not by half.
But that amount of fruit in a week seems insane.
Fresh strawberries, mangoes, blueberries are very expensive. Get them to eat more bananas, apples, pears etc. Or more vegetables like carrots and cucumbers instead. And frozen fruit is fine - if they are eating it with yogurt, you can defrost it in the microwave to make a kind of warm sauce - it's lovely with yogurt. Or make a smoothie. Or try tinned fruit.
Don't buy individual yogurts, that's so expensive - buy a big family pot and spoon out a portion into a bowl. The Aldi greek yogurt is delicious with frozen fruit and seeds added.
I think it's not so much where you are buying your food, but what you are buying, and the quantity of it.
We're a family of 5 adults (me, DH, 3 kids in their early twenties) and we spend around £130 per week on food, including basic toiletries and cleaning products, but not alcohol. We eat carefully and well, and shop in Aldi, Lidl and Morrisons.