We were in a similar situation almost two years ago, my Dc were beneficiaries in a cousins will, with DD the only beneficiary under 18, (she was 16) so solicitors transferred trusteeship to us.
Some money stayed in an NS&I account, just changed the trustee names.
After much research, I opened a savings account with Skipton BS to put the remainder in, then it was withdrawn monthly to drip feed into a S&S ISA that she already had.
The problem that I found, was that not many childrens savings accounts, paying good interest, catered for 16 and over, and of course they cannot have an adult account. There is definitely a gap between 16 and 18.
Depending on the amount of inheritence, and whether they already have ISAs, then maybe consider that as a long term goal, they can only put in 4k a year under 18, 20k after 18.
DD is now 18, so we just moving her Skipton money to an internet savings account (less interest but more will go into ISA monthly and we will then also move the NS&I money too.