If they are paying privately then yes they can employ whom they like and if it is live in then it always below min wage because of the live in bit as accom and food are provided as part of the job, if you are both doing say 60 hours a week then min wage is 5.50ish an hour normally live in is 24/7 and the live in is entitled to a break of about 4/5 hours a day, the going rate through an agency is about £450/£550 a week this depends on the agency, also, and if social services are funding the live in, directly or not, so even doing 84 hours a week at £450 is 5.35 under min wage, + accom which you would pay about £70 a week for and food £60-80.
If you work out how many hours your both going to do, how much your going to be paid and how much accommadation and food they would provide and break it down this way to see how much your income will be and put that down in the letter off your grandparents,
So wage+ accom+food = 92 +70+ 60 = 222
Child benifit = = 18.
Tax credits = = xxx
Then total them together
You can do this as a self employed person or employed but remember tax and NI contributions also.
Hope this is not to confusing but I hope it has helped remember that if your living in that the accom is part of the wage.