That's basically the concept of an au pair but they do count as employees (workers if you want to be technical, but same difference under employment law)
That means they get a wage - not subject to NMW as live in - and a contract, paid holidays etc. If you pay them over the threshold you need to factor in tax and NI but most APs don't get paid that much. Pay depends where you are, rural locations might need more perks to attract and keep an AP!
- How many hours work a week would be acceptable?
Most au pairs work around 25 hours a week, 5 hours a day and not weekends. They can work more, unless they're from Romania or Bulgaria, as there's no legal restriction. A max of 35 hours as a general rule, otherwise they would have no time for English classes! They could do more in the holidays but it would be courteous to pay them more.
- Do they do the same things a nanny would (e.g. school pick up) or is it just meant to be help while you're there too?
Depends on the capabilities of an au pair. Most are capable of doing help with morning routine, school drop off, some household chores a nanny wouldn't do, school pick up and preparing a simple tea. As a general rule au pairs shouldn't have extended sole charge of young children.
- How much do you pay them?
Where are you? Anywhere between £60 and £100 a week, depends on experience, hours and any other perks.
- How do you go about finding one?
You can look by yourself on greataupair.com, aupairworld, nannyjob, gumtree etc. Or you can use an agency. Even if you use an agency check their CV and references thoroughly yourself and do double check the legal advice agencies give as some of them are very suspect.
- anything else you think I should know?!
Don't think of it as a cultural exchange. You will have an employee and a live in one at that. Are you prepared to have another adult/teenager in your house? Younger au pairs can be quite unreasonable and need a lot of hand-holding because they are still quite young. Look for an au pair who has some experience with children your age such as youth groups and not just babysitting and who's had a job before. French ones may have worked as an 'animatuer/animatrice' which is GREAT experience. Test their level of English in writing AND on the phone.
You'll want a good handbook and chores list so the au pair has something written to refer back to. You can't always rely on them having understood what you've said. If it's written then they can look up things they don't understand in a dictionary. It's also worth setting aside a while to show them HOW you like things done.
Good luck!