It's never too early to add prepared pinkies to the feeding plate - most of my garters would be feeding solely on rodent within a couple weeks of birth. Prepare the pinks first(much easier to do when they're still frozen) and then chop up some smaller earthworms and throw them in the mix. The movement of the worm bits will draw their attention and once they start feeding, they usually just clean the plate. I'd feed every 2-3 days, reducing the amount of worm until nothing but prepared pinkies was offered.