hi , i'd consider canjos or making your own boxes , or using a craft box (they can be pretty cheap on ebay sometimes , more so in bulk . (some parents / institutions may shun tobacco products use in kids projects ) ie:cigar boxes . hence the consideration.
gitty's new diy box may be good for you too .. bout the same price as cig box but broke down you'll save on shipping ?
hope that helps , good luk ;-)
How old are the kids? Keep in mind, the younger they are, the shorter their attention spans...
LOTS of simple, cheap, creative and FUN instruments for kids to make:
On the cheap...make fretless diddley bows with cardboard box resonator, "stick-on-face" style like our sponsor's Complete "Pure & Simple" Cigar Box Guitar Kit.
Use tuners like CB gitty's economy open gear tuner, $8.30 for a set of 6, this lets you drill a hole that's one size through the headstock, (closed tuners usually need a shouldered hole, bigger on the back than the front).
cut a shallow slot and use a small/medium finish nail for the nut so it can be fingered like a fretless bass, a shallow slot and a thick finish nail for the bridge to keep the action low for fingering instead of needing to provide a slide for the kids.
depending on the age of the kids, their attention span, and the museum's liability insurance on letting visitors use power tools, you may want to do only a little or a lot of prep. pre-drill the tuner hole and string anchor hole, let them use a triangle file to make the nut and bridge groove, short wood screws for attaching the cardboard box are not too hard to put in by hand if you let them use an awl to start the hole.
2 screws through a cleat or big washers to attach the cardboard box to the back of the neck.
Shoe boxes (local shoe store may have lots of empties), keyboard boxes (check the IT department at work for empties about to be trashed), amazon boxes for not-so big stuff (check friends and family), the museum may even have cardboard boxes to donate.
I agree with Pick. Canjos are the way to go. You can use any kind of can. Different sizes mean different sounds. Depending on the age of the kids, you can pre-drill everything and just have them assemble.