I sent my parents a prepaid US SIM-card in the meantime...The reason, why I think the kiss and fly might be the better choice for me, is that I do not have to expose myself to the terminal-curbside-craziness on a Friday afternoon.
The SIM probably cost you more than $2 that parking would have. Plus I hope their phone has US frequencies and they know how to change the SIM. Plus they have to drag their luggage to the train and then Kiss&Fly.
What craziness on Friday afternoon? It is the highways that are crazy (I-190 and then whichever one you take), not the T5 parking lot. Unless you live right by O'Hare, you will have to deal with the highways anyway. (When I-90 is choked, I usually take Cumberland north, then Devon east into the city. Probably doesn't take less time, but the drive is more pleasant.)
Anyway, good luck, and I hope that all goes smoothly.