Boxes for individual sets What are good boxes (in USA or importable) that would allow young (5 years) Lego builders to store single sets, keeping all the pieces for single build together, while keeping younger sisters (1 year) from exploring?
I've been parting out some of my old Technic sets for my son to play with, and I've found that tackle boxes work nicely for individual sets. It doesn't matter too much which brand you go with. Most of the major brands offer a variety of sizes. Here's a set I just parted out using a Plano 3700 series box:

I also like the parts bins with removable internal sections for storing sets. These are great while building as you can take the bins out and place them in convenient places.

(source: harborfreight.com)
Stanley makes great bins like this, but I've found the cheaper knock-off bins work fine for LEGO at about half the price.
If you are looking for a free solution, I sometimes use pick-a-brick cups or plastic jars such as the ones peanut butter is sold in for storing smaller sets or groups of parts.