It is important to note though that neither you nor the au pair are 'locked in' for any length of time, this would be unworkable, and 2 weeks notice is usual.
If you require trained child care or if "cultural exchange" and some degree of mentoring is of no interest to you (or you are simply too busy for it) it's best to go for another childcare option. (For trained live-in child care a nanny agency may be able to help.)
Although the au pair may have less life-experience than you they must be accorded high status in your household and their reasonable needs must be placed on a par with your own requirements, which may involve certain concessions on your part that you might not wish to make for a professional childcarer or domestic staff.
Au pairs expect two full days off per week and must be able to attend English lessons that are convenient to get to (2-3 hours per week will normally be acceptable in the current climate).
Au pairs won't normally have had much childcare experience and won't be expecting to have to cope with children who aren't easy to deal with or be left in sole charge of under-twos.
Au pairs don't normally work for the agency that introduces them to you (although you should check this) and so au pairs aren't "agency staff" in that sense.
The Home Office au pair guidelines seem to have now been withdrawn with the introduction of a different scheme to encourage youth mobility but it is still clear that having a visitors visa does not normally suffice for those taking up au pair positions.