The kids will chose their own preferred member of staff. DS2 has a distinct preference for a trainee member of staff. She doesn't fill in his learning journal, but in all other senses, she knows him best, and can comment on his day and progress.
I'd judge on the general feeling of the staff, as turnover can mean your key worker can change.
My LO is due to start nursery in the next few months. The nursery appears to tick all the boxes, but the key worker for my LO will be starting around the same time as she starts, and therefore I can't meet her before signing up. The manager said that all of them are qualified (level 3 in childcare). Is it risky putting down a deposit and then LO doesn't settle in or I feel the key worker isn't nice/competent?