OK.... Im also a nanny and this is MY opionion!
We look after the children and their needs etc monday to friday (typically) The parents look after the children saturday and sunday...
BUT i feel life is give and take as well.... so the fact that my boss and the kids may make some cake / biscuits / casserole / soup etc at the weekend that I then eat / serve to the children, means that i would be as happy doing the kids washing from the weekend!
Equally i put our stuff in the dishwasher... then they put theirs in and they may unload it all OR I may unload it all...
It all balances its self out in the end to so im laid back and tbh i dont mind at all! Lifes too short...
HOWEVER i have known of some VERY fussy employers... who make sure the nanny always does EVERY bit of her duties, without helping out etc, and if i worked for a family like that (unlikey that id take the job in the first place!) then i would not be soo happy about doing the kids weekend washing IUSWIM!
However all my bosses have been great... life is give and take, and if it helps the other out then thats fine... (yes they may leave us casserole so i dont have to cook, but i always do a lasagne big enough for them etc!)
Then there is the environment WHY put on a half load of weekend clothes? I would rather save some energy and do it all together regardless!