If I want to buy a microscope for my kids to be able to view single celled creatures and blood cells, about what magnification is required? A Celestron Pentaview digital scope claims up to 600×. Is that going to be sufficient? I somehow get the impression that I'd need 2000× or better, but I'm having trouble verifying any information about magnifications via my good friend Google.
Depending on how much detail you want to see, 400X (as Chris commented) is definitely sufficient. Remember, the lens(es) under/over the stage are labeled 10X, 20X, 40X, etc., while the eyepiece is generally 10X or perhaps 20X (multiplying the two together gives the final magnification). If your target magnification is 400X, then get a 400X scope - it doesn't need to be rated any higher than the highest magnification you want. 600X sounds nice, it's actually higher than the (non-digital) scopes I routinely use for examining mammalian cell cultures. 400-600X should give a very clear detail of both red and white blood cells.
Just my 2-cents worth as visual add-on to @MattDMo 's answer:
Blood smear showing red blood cells and two white blood cells at 400x. Source: Microscope Master
Human red blood cells 1000x. Source: Wikipedia
Human white blood cells 2000x. The small dots (red arrow) are Diplococcus gonorrhea bacteria (Neisseria gonorrhoeae), each ~0.5 micrometers in diameter. Some of the neutrophils have phagocytosed bacteria. Source: Waynes World
Red blood cells visualized by scanning electron miscroscopy. Source: Pinterest.
Note: for illustrative and comparative purposes only; electron microscopy is not the most advisable method for home use.