When registering your kids for cricket in PlayHQ, you will have to create a cricket ID this is different to your MyCricket account.
When you get to the step to link your cricket ID you will follow the link to create a cricket ID for yourself(not your child) using the same email address as you used to setup PlayHQ account.
You will then use the same login to play HQ to register any additional kids and it will recognise the link to the cricket ID you have setup.
If you are still having trouble ring 1800 CRICKET who are also there to help any time.