We have continued our travels. The 10 year old is currently in Washington DC. The 8 year old is in southern Michigan while the 4 year old has been travelling in New England.
Here are the running totals:
10 year old - 1741 miles, 10 states/provinces, $687.17 trip costs
8 year old - 1778 miles, 12 states/provinces $936.60 trip costs
4 year old - 1742 miles , 10 states/provinces $867.78 trip costs
The 10 year old has been the most thrifty traveller, but the 8 year old has hit more states and provinces. The 4 year old has met the requirement for Canadian provinces so she will be travelling throughout the US now.