And (yes, my fault) the scenario came true when one kid went over by some 100MB causing a $40 charge to my credit card 4 hrs before the plan reset. So 5 hrs after I purchased a 1GB block of data it's just gone.
I feel for you. Do you have a device with a data limit built into the OS? My LG volt and GPad have it, a hard shutoff (disables data outright) which I set a shade below my quota in case there are accounting rounding errors, there are probably android apps (didn't look since the LG has it built in) that warn and/or shut off with imminent exceedance. On iOS I like My Data Mgr which lets you set daily and plan monthly limits (no hard shutoff though, but maybe not surprising since iOS doesn't like handing control of settings over to anything outside the OS).