I live one mile from MDW and have been there several times. I beleive you have two options...
1. they arrive at 2pm..you leave at 4pm. I would think their flight ( if on time) will arrive 15 minutes or so early. Find out their flight number, then look on the board at MDW and wait for them by their gate.
2. You can arrange to meet in the walkway that hovers above Cicero Avenue. I think your best bet is for you to get there early and meet them by their gate the plane deboards.
Also, the parking garage at MDW is very accessible. you will probably get charged for at least 2 hours stay. But you can just tell them what level the car is on and that should be no problem. After getting luggage they can just exit to the parking garage.
As far as time goes...it really shouldnt be a factor. If you do have extra time, i would suggest grabbing a bite to eat at the Illinois Bar & Grill. VERY GOOD. Big burgers...big enough that you can split with your other half.
Good Luck hope this helps.