A back pack is your friend! Also, will your two year old be in a car seat? Think about how you are going to carry that on-board. I have a huge car seat backpack that I use to carry the car seat. I first put the diaper bag messanger style over my shoulder, then the car seat backpack and a regular sized backpack over my front. I can then push a stroller for the two year old or she can walk holding my hand and the two smaller bags can ride in the stroller.

How are you going to strap your 3 year old in his/her plane seat? Have you considered a CARES harness? Depending on size, your 2 year-old might also be able to use it.

Will you need car seats where you are going? Your 3 year might also sit in one on the plane but that is a loooong flight to be strapped in like that. A foldable luggage cart or a pull along car seat might help if you need two.