I would just wait until you get there. It is $100 off the adult tickets. Not sure, but I think the child's pass is discounted $75. You will need gov. issued IDs for the adults.
The only advantages to purchasing in advance are to lock in the price (if you think Disney will raise prices before you arrive) or to spread the cost out over several months. Some people like to buy one pass every month or two to avoid one big expense all at once.
If you buy in advance, you still have to go to Guest Relations to exchange the voucher for the actual pass, so there is little to no "line time" advantage.
Currently, the adult DVC discounted AP is $370.62, and the child's is $326.96. The adult DVC-discounted PAP is $483.51 and the child's is $426.00. Those prices include the tax.