Had some time today to put a few touch-ups on my project. I added a DB9 connection (currently using only 4 pins ... I may expand it in the future).
I've also modified some code so that it only turns the LCD on when the driver is seated. The sensor is still functioning but it draws less battery this way. I don't think the baby would want to know weather he is seated, the car temp, and the time until he may be in danger anyway. That's more for the dad to know.
I managed to use my car inverter to run a soldering iron and a hot glue gun ... I did notice a drop in voltage though over the 5 min of use. I don't recommend doing this for too long without your engine running.
Now I just need to work on the menu options.
I've even done a quick calculation that the arduino would last a few months, basically sipping battery, while sensing. So not too worried about battery usage at this point.