OP must not have a credit card or must have one with a very low credit limit if they can't float a refundable plane ticket for a day or two. One-ways on SWA departing MDW tomorrow can be had for under $200 per person. I found those ballpark prices for flights to MSP/DTW/STL but there may be other options. The destination doesn't really matter if you are simply going to cancel the ticket. But in those cases, you would only need an extra $400 or so in "available credit." Unless your credit cards are maxed out that shouldn't be a problem for most folks.

I think when the OP said they couldn't afford a one-way refundable ticket, they were responding to my admittedly cheeky suggestion to surprise their son by flying with him to his destination. If you're just going to get through security to visit him, then the final cost doesn't really matter unless your credit card status would not permit that.