Since the beds will be stained dark, and will take some punishment from my kids, I am not sure what kind of wood I should use. I want something that I can get really dark and relatively uniform, but will also be hard enough so I don't get a glaring chip/dent/etc. every time the boys bang it with something.

I have done plenty of wood construction, but not much furniture grade finishing. So, I would greatly appreciate some advice on 1) the best species of wood for this application, and 2) finishing techniques to get a nice even dark chocolate color.

Of the easily obtained woods:

Maple would be the best wood for hardness and ding resistance. But one of the more difficult to stain evenly or dark.

Poplar can be stained very dark and even, but is soft.

Oak or ash are hard, but have such differences in grain, staining to an even color will be almost impossible.

I did not include pine/spruce/fir simply because those are the hardest to stain evenly and the softest.

Your best first step would be to get a piece of all four types and some black wood dye and dark stains and experiment.

I would look for a better quality "stain" than just picking up the ubiquitous yellow can of Minwax at the big box store.

Beds, like chairs, can be tough to finish because of all the intersections. A wipe-on varnish would be a good choice. Many coats, but less build on each and less time between coats. After drying overnight, lightly sand to remove blemishes and start the next day's finish regimen.

As for processes, I'd refer you to Bob Flexner's "Understanding Wood Finishing" (2d edition) and/or Jeff Jewitt's "Complete Illustrated Guide to Wood Finishing."

Jeff sells TransTint and TransFast dyes from his Homestead Finishing company. Available directly from him or from Rockler & Woodcraft. W.D. Lockwood is the old line dye vendor. There are other dye vendors out there, too.

But the real key is to do samples before starting to finish your project. The best surprise is no surprise.

hardwoods are not normal;y sodl as 'diension lumber.'

They are sld in random leghts and widths to a grading standard for defects, in a given thickness expressed in quarter inches.

4/4 9one inch ) 5/4 1.25 inch thick), 6/4/8/4/12/4, etc
It is also rough and not surfaces.
A 4/4 board should net out at least 3/4 inches after thickness planing,sometimes 1/16 more.
Most furniture hardwoods are much stonger tan softwood lumber, so you wil not see a lot of 2 inch thick anything except in bed rails and a few other heavy structural elements.
A twin size bed can easily use 6 to 8 inch wide 4/4 lumber for rails.

Go up to queen and you need some additional thickness (5/4 or 6/4 depending on width).

Bed posts can be as small as 2x2 inch net in many hardwoods.

Pigment stains exaggerate wood figure, dye stains tend to make color more uniform.

Toning is using colored top coats to alter the shade of the wood.

A dark dye should be able to get most hardwoods (except maybe maple) pretty dark.