If you've got access to anyone with a table saw, you can get them to build (or build yourself) shelving out of MDF for cheap. That's what I build quite a bit of my furniture with, actually - computer desk, a few kitchen counters, siderails for the stairs, a spice rack, and so on. Wood glue (I use Titebond II) with crown or finish staples to hold the pieces in place while it dries should hold for the rest of your life and your children's lives.
I like to use 3/4'' MDF, but 1/2'' should work too. No thinner than that, I think - and those are really the only two cost-effective thicknesses anyway. You can pick up sheets of it at Lowe's or Home Depot, and they can cut the 4x8'' sheets into more manageable pieces for free.
Oh, and with a little router work, sanding, and optionally painting it would even look good!