Front yard work with concrete. it is being mixed on site. i assume the dust is likely airborne to and deposited in very small amounts to the back/side yard. could this cause harm to a 4y/o that plays in grass in back/side yard after work is complete?

There is always dust in the air and there is always a risk when you play and raise dust around you. However the body has protective mechanisims to remove and dispose of such foreign substances if the immunity is not over whelmed and if the dust is not very toxic. I cannot speak specifically to the situation you have not knowing the nature of the dust and the extent of the pollution.