Ah OK, that makes more sense.
Have you had a further UC payment since it's been updated which includes the Disabled Child Element?
If not, can you see in your Journal that's there's an entry along the lines of...Change: Children and other people who live with you...? Or have you got something that says "We'll notify you when your Child details have been verified"...?
If there isn't, then I would update as follows:
Home > Report a change > Children & Other People who live with you.
Input date of change as the Start Date of the DLA award.
Click change next to the Child who has been awarded DLA and re-input their details, including the DLA Award.
Click next / done and submit.
You should then be able to put on a Journal message requesting backdating of the Disabled Child Element and the Carers Element. Just refer to the fact you notified as soon as the Letter was received.
Also...
If your Tax Credit claim closed AFTER the start date of the DLA Award - contact Tax Credits and ask them to reassess your previous award on the grounds that DLA has since been awarded.
When you get a revised TC notification, add a message to your Journal asking for your Transitional Protection to be reviewed on the grounds your prior TC award has been revised.