A bit of misinformation here:
Au pairs are employees. That means contract and 5.6weeks paid holiday (can be pro rata as they rarely stay 12 months). Despite being live in, and therefore usually below the threshold, it also means operating PAYE and deducting tax and NI if you pay over the threshold or if it's their second job.
Pay is £65+/week depending on location, duties, quals/experience etc. Expect to pay a premium for driving, experience or excellent English. Most people provide a cheap phone mostly for work use, some pay for English lessons. If you need the au pair to travel to school/activities you will need to insure them on your car or pay for a bus pass.
Besides Romanian and Bulgarian nations there are no restrictions on hours (buy should be remunerated appropriately if working more than 'standard' hours) or duties, including age of children, but be realistic in your expectations and their general competency.
Some light housework is pretty much par for the course.
Before you recruit make a list of what they'll be doing and what you want and at stick to it. Have non-negotiable qualities, nice to haves and deal-breakers. Be ruthless!
It's infinitely preferable to sort everything by the book and then develop a nice, friendly/familial relationship than not to it and be threatened with an employment tribunal when things go sour. It does happen, happened to a MNer a couple of years back, but if you respect the rules you will be fine. 99% of the time it's a safety net and everything goes swimmingly (in terms of legals).
By their nature most are unqualified, relatively inexperienced and need a lot if direction. The clearer you can be at the start of the process about duties, expectations, benefits and house rules the better you will cope with odd questions.
Good luck!