Unfortunately the disabled exception applies only to the person that has money in the Plan.
So there is no exception for a disable child for your 401K distribution.
For more detail see below:
Exceptions to 401K are:
http://www.401khelpcenter.com/401k_education/hardship_withdrawal_article.html#.WJtLoVUrKM8
"But to discourage these early hardship withdrawals, in most all cases the IRS imposes a hefty financial penalty including a 10 percent early withdrawal penalty if you are younger than 59 1/2. You may qualify to take a penalty-free withdrawal if you meet one of the following exceptions: You become totally disabled."