Hi Yorkiegirl,
Yes, laundry and school collection are definately fair game. As a guide I think they work for around 25 hours a week for room, board, and about £60 - £70 per week. Pretty much any house hold chore/light housework/childcare is fair game. But, probably not really long stints of unsupervised childcare.
If you hire someone from the EU I think they just an employee like anyone else, but if you keep her hours low enough that you pay her less than about £90/week then your taxes are virtually nothing. So I guess that would up to I don't know say 35 hours per week.
You aren't required to pay for them to go to language class, but if her English isn't great it mught be in everyone's best interest if you do. It is also a good way for her to get out and meet other au pairs.
Can't remember how old your girls are. They are still quite young, aren't they?