Hi Tom,

Your children may not be able to comment or vote if they are under the age of 13. Discussion features for accounts for users under the age of 13 are restricted due to online privacy protection laws for children and the security measures Khan Academy takes for children's privacy. More information on how child accounts are different than regular accounts can be found in this article.

If your children are over the age of 13 and still have restricted accounts, you may need to update your their birthdays in their settings. Instructions on doing so can be found here. 

I hope this helps! Please let me know if you have any further questions.