How can we keep crows from stealing other baby birds?

We have what we guess is a sandpiper that laid about 4-5 eggs and they hatched about 1 week ago in a low, 4-5 tall tree. This morning, our dog started freaking out & the wife went out to see what the commotion was and observed a crow in the baby bird nest. The other bird was trying to get the crow away by divebombing it with negative results. Our dog must have heard the noise. Anyways, upon inspection, only 1 baby is left. How can we keep the last remaining baby safe but still allow the mother to feed it. We tied a plastic bag near the nest but are afraid the mom will now be scared & don't wanna risk starving the sole survivor.