Hmmm.. Simple question, a short and a long answer.
you must calibrate your monitor.
Calibrating the monitor sets two parameters: brightness (or "luminosity") and color balance (or "white point"). Both of these are important because they influence how you see an image on the screen and are completely independent of the profiling process.
When preparing files for print (as you're doing for the children's book), you want to set your calibration so that white as seen on your monitor in Photoshop (255/255/255) matches the brightness and color of a white sheet of paper held next to the monitor. (This is, of course, also influenced by the lighting in your work area - another topic of conversation.)
Most monitors can have their white points adjusted in a range from 5,000ºK to 9,300ºK. 9,300 is very blue, 5,000 seems very yellow by comparison. A general rule of thumb is to choose 5,000 to 5,500 if the majority of your work is for print, 6,500 if the majority of your work is for web. In any case, once you choose a white point, stick with it, don't change it. BTW, for all practical purposes, D50 = 5,000ºK and D65 = 6,500ºK.
Luminosity is measured in candelas per square meter (cd/m2). Out-of-the-box. most monitors are set to their maxmimum (450 cd/m2, or more). Again, the rule of thumb is to choose a setting between 100 and 140. Of course, the lighting in your work environment will play into this decision.
A common complaint is that prints always look darker than the monitor. More often than not, this is because the monitor is too bright (i.e., the cd/m2 is to high.)
At any rate, I don't know what it means to have your monitor set to "the factory standard Contrast 50 and Brightness 50". The only way to create a good, accurate calibration/profile set-up for your monitor is to use a hardware device like an i1Display 2.
That's the brief version. Hope that helps.
[edit - for clarity]
Just one addendum to Rick's thorough answer:
Calibrate and profile your monitors often and regularly.
Thank you Rick [and Ramon]. I'm going step by step in the "Help me calibrate my monitor" thread. I'm going to need all the help I can get!