Previously I feed the stray cats around my house area with dry kibble,they are more long lasting and not easily spoil if left there.
You can try Blackwood cat food, I think it is more nutritious and lower cost if compared to other cat food in the same range.

One thing to note is how old are the kittens?? If they are not big enough you might want to give them milk first. Not the usual cow milk of cause, you have to use goat milk. For convenience sake, you can feed both kittens and mother with Royal Canin Mother and Baby Cat food but it is a bit pricey. You can mix with water to ease the kitten transition.

Hope it helps and thanks for doing this.

Get. Them. Spayed/Neutered.