First of all, welcome to you and to your kittens. Is there any important reason you are only feeding dry - wet food is much better, or a mix is what most of us do. A wet meal morning and evening at least, with a bowl of dry to keep them going in between, though 7 week old kittens need feeding four/five times a day. I also give my foster kittens a supplement of fromage frais (unsalted cream cheese) three times a week, and some kitten milk or goats milk daily until they are 8/9 weeks old, though this is not strictly necessary. Do not give them cows milk as this is high in lactose and can upset their stomachs.
On the kitten food question, the existence of kitten food is quite recent in the scheme of things - in the old days kittens were put onto adult food as soon as they were weaned. But kitten food does give them a better start in life - it is balanced for growing bodies and sensitive stomachs. It is also made in smaller pieces, easier for them to eat. I would keep them on it until they are 6 months, gradually starting to switch over at 5 months, adding the adult. Some people might say longer. If they seem to be very hungry, start earlier.