At a mother and toddler group I used to go, the health visitor that ran it, gently told off a mum for giving a toddler irn bru - and then went on to tell us that one hot summers day she did a home visit to a young mum and found her giving irn bru in a bottle to a six week old baby. She hid the fact that she was horrified and asked why she was giving her irn bru to drink, and the mum replied that the baby was thirsty but wouldnt take milk. The health visitor asked her why she didnt give the baby water, and she replied that that would be cruel. The health visitor put her straight.
I think there is a bit of an element of that with some parents - until quite recently in Scotland it was considered child abuse not to constantly give children sweeties.
Processed food frequently appears in BOGOFs, creating the illusion of saving money; they are attractively packaged and endorsed by all kinds of little "healthy eating", "especially for kids" badges. On the face of it it seems like a sensible purchase - its endorsed by a major supermarket who says that it is ideal to serve to children; it "saves money", children want it.
As a parent its really, really difficult to try to balance things in the face of the marketing constantly pushed at you - remember the sugar laden Sunny Delight, being pushed as a healthy drink for children? For ages I bought these little packaged rolls of what I then thought was condensed fruit (cant remember its name now, comes in a blue packet), before I realised that it was basically a well packaged sweetie.