Under 22 on the date of the application or if over 22 in continuous studies from before the age of 22 and those studies must continue to visa issuance.
If the child is over 22 then they must have been in school since before the age of 22 and must still be in school now. Once the application is filed, they must remain in school until the application is fully processed and the visa issued.
If the child stopped or stops going to school at any time, they no longer qualify to be included in the application.
@harshul3311 : Your children should be of below 18 age when you apply or otherwise going to school and till the process is over (which takes more than 3 years) he/she should still be school going otherwise not accepted !