I have not had to deal with this situation in Thailand, but if the minor child is nearly an adult by age, it might makes sense to wait until the child reaches the age under which he or she can give a valid receipt. Otherwise I agree with Jane Huntley.
If push comes to shove, you can wait until proceedings are commenced in England, which from a Thailand perspective may not make economic sense, and may well force the parents to agree to the appointment of Trustees to hold on to and invest the monies until the minor child is able to receive the monies.
From the nature of the original question are the parents able and willing to invest the monies for the minor child or is there a risk that the parents may use the monies so that there is nothing for the minor child to inherit in due time?
I know this might be more practical rather than legal advice, but sometimes practicality is needed.
Peter Double / Probate Resealing Services