Do baby robins need to be incubated after they hatch? For how long?

Nope. You will need to make sure they are warm enough, but no need to incubate them after they hatch. What you can do is hang a 40-watt light bulb over the newborns (not too close though), or place a heating pad, on the lowest setting, beneath where you’re putting them. And as crazy as this sounds, you can also fill a rubber glove with warm water, or heat a towel in the microwave for approximately 25 seconds, and place either one in where the bird is for warming purposes.