This article is here to help diagnose why your child is missing from your account, and why you might not be able to add them. If you're looking at how to add your child, there is an article for that here.
Please note: Your child will need to be added to your account before being visible. If someone else has your child in their account, a request to share the child will be sent to them.
1. Your child exists in another account.
This is the main reason why a child cannot be added to an account. To avoid duplicate applications, Dash only allows a child to exist once. If you try to add a child that already exists, you will get this error:
A notification will be sent to the person with the child in their account, and then they will need to approve secondary parent access from their dashboard before you can move on.
Try to identify if someone else might've processed an application for your child in the past, and ask them to check their Dash account for a notification.
2. The child has been hidden from your account.
If you've had your child in your account previously, but now they're missing, it could be that you've hidden them. This is similar to deleting your child from your account, but all their information remains in the system.
If you've completed an application in the past then at some point your child was connected to your account. You can check if you've completed an application in the past by flicking the 'Show recent only' switch on your dashboard, as shown below. 
If you believe they are hidden, and you'd like to unhide your child, contact Dash Support at support@dashhelp.zendesk.com.
3. You've found your way to the incorrect instance of Dash.
Please check the url of the page you're on. If your child goes to a government school, you should be on:
https://nswschoolsportunit.dashapp.com.au/dashboard
CSNSW (Catholic schools) and CISNSW (Combined Independent Schools) Dash instances will cause problems if you're trying to log into an existing NSW School Sports Unit account.
Please note: If you don't have your child appearing in your account under 'My Children', you won't be able to process an application until they're added.
If you're having troubles with this then please reach out.