Yes, fast food once a week in moderate quantity won't do any harm to your kid. You definitely can. But few suggestions being a certified child nutritionist will suggest you: Try making your normal food innovative and tasty. For eg, if your child craves pasta, you can replace the sauce made with refined flour(maida) with wheat flour, add in beetroot juice to make it vibrant pink. Trust me your kiddo will enjoy it without even knowing it is full of nutrients. You baby will be happy and so will you. He won't ask for fast food if you make your home food creative, I bet. Lastly, it is fine to give them weekly their desired foods it won't harm your baby. I hope your query is well answered. Thank you, have a lovely day.