Join Date: Jan 2006
Location: Silicon Valley, CA (SJC, SFO)
Programs: UA 1P, AA, *A Gold, Hyatt GP, HHonors, SPG, Priority Club
Posts: 172
A handsfree bag like a backpack is definitely best, as you'll need your hands for other things! But a "real" diaper bag is not necessary - we're currently using (for a 3 YO and 7 month old) a bike messenger bag from Timbuk2, with a waterproof changing pad salvaged from an old diaper bag (or you can buy these separately).
I agree with the PP about transporting milk and liquids - minimize what you have to bring and buy what you can after TSA. If you have to bring (and chill) breast milk, use some of the small, padded cooler bags that either come with a breast pump or from formula companies - or any small cooler bag. If you have liquids or a cooler ice pack, you WILL need to declare it and/or go through the family/medical line at security (depending on the airport), but you should be OK to take it through.
As for leaks: ziploc bags are your friend! if it's not feasible to put all the messy stuff in ziplocs, you can do the opposite and put the clean stuff (extra clothes - you are packing some of those, right? - books, papers, etc.) in ziplocs to be extra sure.
Hope that helps - good luck! And definitely browse this forum for lots of other tips on flying with infants and toddlers.