I always pack in ziplock bags. The gallon size work great for little ones, and I use the 2 or 2.5 gallon size for myself and my bug kids.
Another trick: I pack one carryon (since we travel with 5, we always have extra carryons from the kids, since they just carry a backpack) with a change of clothing for each person in their own ziplock (shorts or pants, depending on season, t-shirt, socks and underwear). I pack a second ziplock with a swim suit and pajamas, per person. Another ziplock carries our toothbrushes and my contact lenses stuff. If our luggage is delayed, it gives us what we need to get by. We just put this bag up in the overhead compartment.
A Funny Story: During our last trip to WDW, in April 2001, I packed all the kids clothes in ziplocks. DD and DD chose their clothes each day by looking at the t-shirt and choosing what they wanted. My youngest DS, who had just turned 4, chose his clothes everyday by his underwear (Lion King, Superheroes, Pokemon, etc) I was much amused.
Julia