What Do I Feed My 6 Week Old Border Terrier?

When my Boston was a puppy, I fed her Mighty Dog canned food and mixed it with the soft Gaines burger type dog food.  Their teeth are so tiny they have a hard time chewing the hard food.  Once she got older she wanted to eat what my 2 black labs were eating and now eats regular dog chow hard food with no problems.

I would not use soft foods as soft foods cause tartar which tartar causes stinky breath.
If you use soft foods brush their teeth morning and night until on hard food.
My dogs are on hard food now and I still brush their teeth every night.
Tartar is hard to get rid of so try to prevent it the best way you can.
But don't give them soft food for a long time. Start introducing hard food into their diet once they get old enough. Plus Hard food will help them lose their baby teeth! If they are already loose. :)

I would recommend the james well beloved pouches for your pup or the nature menu foods.  After a long time of trying out so many things I took the time to study what's in the foods.  If you read the lable on the nature menu you can actually see your feeding your dog meat compared to baker, beta puppy etc that are 4% meat so your dog will always be hungry as they need meat in their diet. 
It really pays off in the long run to acutally be giving them meat and wish I had learned sooner.