Will trust - possible to change trust or deal with adult interest?

We have a will trust which left land on trust to the deceased’s three minor grandchildren on a bare trust with their parents as trustees. The oldest child is now over 18 but has had mental capacity issues due to mental health/drug struggles. The parents (who are the trustees) have a strained relationship with the adult child.

They are keen to protect the adult child’s interest in the land and want to know if they can continue to hold it on trust until the adult child is well enough to deal with the land or proceeds of sale. They also want to protect the minor beneficiaries’ share.

Do the trustees have the authority to put the minors’ shares into a discretionary trust to protect their interests? Do they need authority or permission from the adult child? Or do they need to apply to the court (and if so how is this process dealt with). The will includes the standard STEP provisions (2nd Edition) but not the special provisions.

Any thoughts or ideas welcomed.