In addition to the issues covered by Paul Saunders, I would be very cautious about advancing monies etc to the parents r to the minor beneficiaries, until we know more about the State in which the Beneficiaries are resident as the local State law will apply to such advances, and may well have local restrictions on such monies. For example many years ago a child film star lost her monies because of mismanagement of monies she earned, and which were supposed to be invested carefully and for her benefit. Therefore the local Sate laws need to be considered - especially in California.
This also leads into the risk that the USA being highly litigious, the UK Executor / Trustee may be drawn into litigation in the USA if the monies are not properly administered for the benefit of the USA minor beneficiaries.
This certainly needs further investigation, and the UK Executor/ Trustees may need to have an indemnity from the USA parents / guardians, and perhaps even insurance. There may even be local court supervision needed of the investments.
I know this is practical advice rather than legal, but having practised law in the USA, caution and advice from local lawyers will be a must.
Peter Double / Probate Resealing Services