A 'slow' puncture, or maybe a leaking valve might well not have gone flat overnight, as the car was stationary. Was the valve cap still there? In my experience kids generally dont bother replacing them after letting down the tyre. Test the valve with some spit of soapy water, see if you can get a bubble out, then check the tyre carefully for nails etc. If you have an alloy wheel air could also be leaking past the rim, if it is even slightly corroded, this will also tend to happen faster while the car is in motion.