The screen and keyboard are too small to make them an effective learning tool.
They have a reputation for being very slow especially with Win XP/7 and antivirus software.
Eye strain and cramped shoulders, headaches and neck strain will increase from the tiny screen, keyboard and mouse pad.
Any extended use -20 minutes or more -the netbook should be positioned in a cradle to try to mitigate these issues - another expense (OH&S guidelines from Worksafe Victoria)
How do you stop a child who has their own laptop with music, photos and homework etc from NOT taking it into their bedroom?
When it is not a communal computer for general use but a personal one it will encourage continual use which is not what netbooks are for- with resultant health issues