I assembled the charging and battery monitoring circuit and tested my circuit (minus the XBee radio, which I don't have yet) and checked out the low voltage warning as well as current draw. The Sparkfun Uh-Oh battery monitor is super simple to use - just adjust the trim pot to the voltage cutoff you want and the big green LED turns on when the battery voltage drops to that point. It's possible to solder a couple of wires to the Uh-Oh board and connect it to the Spark Core and have the Spark Core send my phone a message when the battery voltage drops but really that's kind of overkill- the big green LED tells me everything I need to know and it'll be easy for my kids to see when using this for their projects.
I also checked the current draw and it's just under the specs on the Spark Core data sheet since I was getting 280mA draw with the OpenLog writing at full speed. The OpenLog draws around 6mA at full speed. The XBee Pro is supposed to draw around 215mA so I should see a bit north of 500mA total current draw when the circuit is fully assembled. The Pololu 5V boost regulator is good for about 700mA so I have a little bit of headroom there. Right now the battery monitor is set to turn on at 3.2V so I may play around with that a bit once the circuit is finished. Here's the current output vs. voltage input graph of the Pololu 5V boost regulator.