I think once a week would be the max I would want my kids to eat fast food but we do have food out after swimming lessons on a Saturday most weeks. Sometimes it is mcdonalds but sometimes it is pub grub, pizza, Chinese noodles, a sandwich in the leisure centre cafe.
My kids have something unhealthy almost every day probably but one day it will be crisps, another chocolate or sweets, another a cake another a biscuit and so on. They don't have them all on the same day. They eat healthily the rest of the time. We don't have frozen/processed foods for dinners and make most meals from scratch. They eat fruit and veg and have a varied diet. The dietician we saw for Cadan said he eats really well for his age and I was completely honest. Maybe I am fooling myself but I tend to think of us as middle of the road when it comes to eating. We don't deprive them of the sometimes foods but we don't overindulge and we have healthy food most of the time. I don't know a single child IRL who doesn't eat some sweets or cakes etc. I see that as the extreme end of a spectrum with kids who eat chicken nuggets and chips every night with sweets and crisps every day being the other end of the spectrum. DH's friend was brought up not really knowing what sweets and fizzy drinks were and then went mad on them when he had his own buying power as a teenager. I wouldn't want to end up in that situation either.