Hello there,
So there are loads of companies who are happy to look for a holiday nanny for you. I have been working with children for 16 years and a little while ago I started hellomums.co.uk. We offer police checked and paediatric first aid certified babysitters who all have many years of experience - for many of them this is just a little extra as they work as teachers, nannies, maternity nurses and so on and so forth.
I have written all this down so you know where I'm coming from
I would say it is a 50/50 scenario if you don't know the person who is going with you as it can go amazingly well and obviously it can all go pear shaped, too.
Regardless of how you end up choosing the holiday nanny I would suggest that you arrange a couple of babysittings (can be over the weekend during the day when your nanny would not be there but you are) where you can all spend some time together so when the holiday comes you don't feel worried or concerned about leaving the nanny with your little one.
Obviously if your nanny has a recommendation of a another nanny that could be the best option for you, but most agencies specialising in holiday nannies are a good option as well.
Good Luck either way and have a lovely holiday:)
Kate