Self Assessment for child

I have tried registering my children for a Self Assessment online, but it will not allow me as the applicant is under 15 years and 9 months. I have since got their UTR number and NI number. I have then sent off a paper tax return as I was unable to do online. This has just been returned, saying that "while it is ok for a mother to sign, we do not have her as a third party on the record." There is no information what I need to do on the letter about how I add myself as a third party - could someone advise please? They have given me a deadline of returning the completed tax return by 8 May. I have been on the self-assessment line now for 45minutes.

Hi,

To be added as a third party on your child's record, please write to us confirming your name, date of birth, and address, and also what you would like authority to do on their behalf.
Both you and your child should sign the letter, confirming that they agree to your acting on their behalf as a third party.

Please send your letter to:

Pay As You Earn and Self Assessment,
HM Revenue and Customs,
BX9 1AS

Thank you.

@jopp, how did you get the UTR and NI number for your child? I need to submit a return for my 10 year old and my 15 year old and can't figure out how, thanks!

My enquiry is similar to the one above. I'm asking on behalf of my daughter. She needs to register her two children for SA as they are both child models. She was told she can register them online as their capacitor but we can't see any way of doing this online. Please can someone advise how you register a child for SA and get a UTR for each of them when they don't have a NI number.

Hi Katherine,
To give you the best advice here, we may need to ask your daughter for more information.
Please ask her to contact us by webchat or phone here:
Self Assessment: general enquiries 
So we can advise her further.
Thank you.