Separate Entry Permit application
Dependent children have their own immigration application
ICA's current Dependent Family Members guidance defines dependent family members as a spouse, unmarried children under 18 and ageing parents, and states that each dependent family member must make a separate application for an Entry Permit.
That does not necessarily mean every child must create a completely separate online user account. It means the child is a separate immigration applicant with their own Entry Permit application and supporting identity documents.