unless the rates to fly are outrageous (and to Orlando form most places anything above $300 is probably too high), driving is going to be a major pain. We just bought tickets on Air Tran for June for 4 persons at the rate of $850.00. It is $100 more then we paid last year but still reasonably priced (we estimate that for us to drive down it would cost us about $600.00 ($100 in gas each way, $100 for a hotel room each way, and between $100-$200 in food/snacks, extras) plus 4 more days to our vacation (or off our stay in Disney), plus 2000 miles on our car.
A mapquest from Boston MA to Disney World indicates its about a 21 hour drive straight through, 1300 miles. At 30mpg (which most cars still don't get.. my little HHR gets 29) fuel costs would be about $130.00. At 23 mpg its about $160.00. That cost is each way, so gas alone will run you $250-$300 total.
With a 6mo I would expect that you wouldn't be able to do the trip in 2 days and still stay sane. I would think you need 3 days of driving each way, which adds another 6 days to your trip (otherwise it adds another 2 days to your trip). This would be roughly 8 hours of driving each day. With 3 days of driving with an average total hotel cost of $100 a night, that's an extra $600.
Then add food for 3 people during the drive at $75 per day (that's $25 for each person for 2 meals a day, one fast food one sit down, which isn't unreasonable unless all you eat is fast food, which then would be about $15 per person per day for 2 meals) for 3 days and thats another $450.
So in total if it takes you two more days each way your looking at either 4 more days of vacation that needs to be taken (or 4 less days at Disney), and a total oop cost of about $825 or for 3 days to drive a total of about $1100 to price against plane tickets.