I too would advise using the mixer to handle multiple mics for the drums, which would then get routed down to a single stereo set of outputs running into your audio interface.
 
This would allow some prehistoric level setting of the different mics, but all of that would be only good to the point of when they are are mixed together into 2 channels from the output from the mixer, so it is by no means a perfect situation.  it IS however a way to sort of control input levels and balancing of each kit piece that was miked.  
 
This is a garage-band approach, but that is what they are doing for now, so it will still help them to craft reasonable rough cuts of their tunes - and likely help them with writing and arranging.
 
When feasible, look into either renting additional mics, or picking up a few reasonable drum mics, a couple of overhead booms, etc., and look into either renting an audio interface with 8 mic inputs, or buying one.  It would be a good investment - I would also suggest that you look for an audio interface that supports adding expansion channels - (usually must be same model or otherwise specially made to support this), so that you can eventually end up supporting up to 16 mic inputs.
 
Drums have a lot of kit pieces, and the best way to be able to manage/control/edit/shape each kit piece is to have as many kit pieces using their own mic as possible.  Some setups use multiple mics for the snare - with a top and a bottom mic, and this is sometimes also done for each tom.
 
In the end, you can only do what your budget allows, and it may need to be built up in a series of layers, adding pieces and capabilities as funding allows.
 
Just remember that a BUNCH of the early rock/pop was done with 4-track tape machines, and that all came out pretty good, so even if it takes time to get to a modern set of capabilities, you should be able to sound pretty good all along the way....
 
Best of luck, 
 
Bob Bone