In the last 2 years, we've taken several long (5+ hour) plane rides with the kids. For anyone who's done this, you know it can be very taxing on all involved: Getting packed, delivered to the airport, passed through security, and onto the plane is stressing for adults, and more so for kids. Add on top of that the excitement of doing something different. Then top it off with the changes to the ability to stand up in a plane and stretch your legs in post-911 plane rides with less food/drink service and you're in for it.
We got a tip from somewhere to take presents on the plane, then slowly dole them out through out the trip. The presents need to be small for portability and usable in cramped airline seats, but still able to hold a child's attention for an extended period. We found that a large collection of toys from your local dollar store works well. We also peruse garage sales for months ahead of time looking for small trinkets. We sometime also get hand-me-down bags of "miscellaneous" toys, especially for little boys. Rather them pass them onto our kids, we hoard them for use on the plane ride.
Course, you need to make sure you wrap them in non-foil wrapping to get past security. You should also wrap in such a way that an adult could open and re-seal in case you get closer inspection at the airport. I suppose you could skip the wrapping, but that's part of the fun!
Depending on your child, this tip might also come in handy during the conference. If you're at a session you really want to make sure you can stay/pay full attention to but you have your child with you, pop out the gift for some instant sanity. Course, at every conference we've been to, there was something in the exhibitor hall that our children coveted, so you can use that as well.
This reads to me like we have very material children, but that's not the case. They just really get into "new" stuff, even when it's not. For younger children, you can use the same trick by stashing away toys they already have and reproducing them at key times, though some might call that cheating :)