When do you feed stray kittens dry cat food?

I had a stray cat have babies in my car port. Shes been taking care of them very well. We feed her and another cat as well (thats all).. I was wondering when do we feed her kittens dry cat food? How do we keep them from going out of the property when they start walking around more? Were going to trap them and get them fixed when they are old enough.