You are correct about the 18 year old. She does not need a legal guardian and if the UK authorities says it is fine, you can take her over to England. She will be able to apply for her own passport and visa and would not need your legal assistance. Meaning, she would not need the permission of her guardian to take the above actions.
For the 12 year old, you have a couple of options. Which one you are going to take will depend on what makes it easier for you to get the child into the UK and for that you need to speak to a UK immigration attorney.
The first option is to apply to the High Court for you to be appointed as the legal guardian of the child. You can appoint an attorney that specializes in family law and is resident in the same area as the child to bring the application on your behalf. It is not necessarily necesary for you to come to South Africa to do that, although the possibility cannot be discounted.
The other option is to do an inter country adoption. This is going to be a lot more red tape than the first option, but in the end it is going to be a lot easier for you (in my opinion) to get the child into the UK if you follow this option.
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