First interviews are very rarely paid in the nanny industry. The exception would be paying expenses or a weekend long interview, which doubles up as a trial.
Some parents will offer to pay for the time taken for a second interview. Whether you do or not is your choice, but if you didn’t give your children the chance to meet your nanny at a first interview then she’s probably expecting to come back for a second interview, and many nannies won’t accept a job where they’ve not met the children first. Shortlisting candidates is fairly standard, especially in a competitive market, and second interviews are sufficiently common that they count as part of the normal recruitment process. A second interview should remain fairly short, although you might ask your nanny to play with your children or join in the evening routine.
Trial afternoons or evenings last a lot longer than an interview, and this is the point where parents start paying. Even though you will be around and observing the nanny a trial is work (and hard work at that!). If you ask a nanny to come with a prepared activity or you expect them to stay more than a couple of hours you should pay.
Settling in sessions, even if they take place before the contract’s official start date, should always be paid at the nanny’s usual hourly rate. While you will still be around the whole idea is that you play a minimal role, and may even nip out the house. Any shared care after the start date counts as a handover and your nanny is contractually obliged to be there, so you are contractually obliged to pay her for all her contracted hours, so settling in can be a good investment.