Sorry to hear this has happened to you, hopefully your son didn't get hurt.
Hopefully you can see and reach both ends of the wires he cut so you can see the wires clearly (if it's hard to see, get a flashlight).
Disconnect the battery.
You need to get barrel connectors, wire strippers and a crimper.
Here's the barrel connector: http://www.grainger.com/Grainger/3M-****-Connector-4X313?Pid=search
Here's the crimper/wire stripper combination tool: http://www.grainger.com/Grainger/3M-Heavy-Duty-Crimping-Tool-1A076?BaseItem=4X313
If you look inside the connector, you will see a piece of metal. This piece of metal will act as a "bridge" for the electricity. The crimper will squish the metal to the wire (metal to metal contact). The crimper has a color code dot in the front of the tool's mouth to show you where the barrel connector needs to go. The mouth has two places to crimp and the correct spot needs to be used to crimp correctly.
Go one by one in the crimping process so you can make sure you get this correct and not cross wire anything. Each wire has a specific color code so one end of the cut should match another end of the cut (make sure you look closely at the wire's insulation because sometimes there may be two colors on the wire's insulation). Everything should be ok as long as you pay close attention to the color codes and crimp properly.
Once you find each end of the cut wire, remove a quarter of an inch of insulation on both sides using the stripping holes found inside the crimper's handle (the quarter inch of wire should show bare metal). Take a connector and place the exposed wire into the connector (take a quick peak to make sure the wire is inside the metal tube), then take the crimper and squeeze the crimper as hard as you can where the wire and metal barrel meet. You should be able to pull on the connector without the wire coming loose. Now place the other end of the wire into the opposite side of the connector and squeeze again. Give it a tug to make sure it is on.
That's it - repeat as many times as necessary to get the ends "bridged." Once you're done, connect the battery and give it a test. You should be good to go. It's a long explanation but this is really easy to do, once you see the connector and crimper this explanation will make more sense to you.