As Jack the Sailor said - there's a legal question and a moral one. Legally, you could get in trouble if, say, your neighbors decided to call the INS, and the INS had nothing better to do than investigate a couple of suspected illegal yard workers. (I think they prefer to concentrate their efforts on bigger operations where they'll round up a large group at one time). Also, what if someone got injured on the job. I presume the nursery would have workers comp insurance that would take care of their employees, but what would you do if someone accidentally sliced off a finger or fell off a ladder?

I think there would be nothing at all that was morally wrong with hiring these day workers as long as you pay them a reasonable wage. When I lived in New Jersey I did some volunteer work with day workers, mostly from Guatemala and El Salvador. They're just people trying to make a living and send what they can back to their families. They'd probably do a great job for you.

The other thing to keep in mind is make sure you know what needs to be done - how deep the trees need to be planted, etc.   That's things the nursery would know, but the day workers will need supervision.