The other lane looks pretty clear, continuing to run across the street is probably safer. I'd do the two arm waving over the head "STOP!" motion while also yelling that and looking really concerned. I'm not gonna get hit by a truck myself to save a baby. My life is worth more to me than a baby's cause it's mine. Hopefully this would cause the truck to swerve and hit the self driving car. More realistically that baby is dead, I'm scarred for life, also will be deeply disturbed that someone left an infant in the middle of a crosswalk and as a witness, help the horribly irresponsible parents sue tesla and the big truck guy/big truck manufacturer