I'm learning here:
1) Anybody else in the truck? Any visible weapons, especially firearms?
2) In Texas, deadly force is not authorized to prevent a kidnapping - only aggravated kidnapping. However, the THREAT to use deadly force is NOT the use of deadly force.
3) Most kidnappers, I would presume, would not park in a parking lot if a curbside place was open.
4) Did the man's speed increase when he snatched the kid? Few parents would need to pick up their kid the way you describe. Those who need to do so probably wouldn't take time to chat. They would walk directly to the kid, pick him up, and return to the truck. If the guy had been moving normally (or slowly), talked to the kid for a bit, then suddenly snatched and ran - that would tend to ring more alarm bells.
5) Above all, get a good description of the kid, the man, his truck (& license plate), and anyone else in the truck.
6) I'd be on the cell phone immediately giving that info to 9-1-1 while yelling at the guy to put the kid down.
7) Talking to the kid was an obvious point that I had missed. Thanks, Long Path.