Contract covers basic employment legislation (and I'd disagree on the pocket money/pay point - it is pay, if you go over the tax threshold you need to deduct tax and NI and if they do extra hours they get paid).
Things like:
How much they're paid, when and how
5.6 weeks paid holiday, including Bank Hols.
A probationary and notice period
Defined hours and duties
A disciplinary procedure and grounds for instant dismissal e.g. child abuse/drinking on the job
Sick pay
More advice here
House rules cover things which are particular to your house - friends, use of phone, showers at midnight etc. Try to imagine your DCs as teens and what you expect, then imagine what you'd expect from someone doing a job (no drinking heavily when you have to be up and chirpy at 7). The clearer your expectations are of behaviour the better you can articulate them. In general think of what you'd expect of your own teens as a minimum and a live in nanny as a maximum.
I'd also them suggest a handbook with information about the daily jobs, routines, children's likes/dislikes/schedules/needs, info on the local area, useful numbers, 'how-to' guides for household appliances, a couple if easy recipes....anything you're going to tell them, write it down and put it in a binder so they can refer back. Even au pairs with fab English will find it easier to have something written and au pairs who don't can look things up in their dictionary later. Plus there's a lot of info to pass on and you don't want anything getting lost.
Recruitment wise write your job and person spec first. Be ruthless for a while and if you're not getting candidates revisit your specs. Don't fall into the trap of trying to accommodate almost-there candidates if you don't have to as you're just creating work fir yourself.
Good luck! When it works it works really well but the key is matching up expectations... Don't try to be something you're not and don't pretend the job is something it isn't.