Hi Lorry:
We use stacking containers-- unfortunately, since we acquired LEGOS over a span of time, not all our containers stack nicely together. Stacking nicely is the ideal.
Therefore-- buy several at once and buy excess capacity.
The bigger the container, the more it holds-- and the heavier to lift and store. Choose boxes with this in mind.
We separate DUPLO and LEGO bricks-- different age groups
If you aren't already using plastic food-service trays for activities-- get to a restaurant supply store and get some now. Endlessly useful (arts & crafts, beads, building, egg decorating, etc) and a great way to insist the actual LEGO building stays contained!
Since kids LOVE the "Dump-and-Rummage" game, we've started providing old tablecloths, tarps, etc to dump onto. Easier to lift up and pour back into the tubs afterward.
Little wheels are the most popular pieces-- don't let them walk away.
When your builders are done creating, you can volunteer to put their creations on display at the library for a set time, before you dismantle them.
Have fun!