agree yabu and yanbu 
as others have said, fair enough you want to know where your dc are
but i wouldnt be happy sending texts, saying can we go to xxx and then wait for the reply
but i dont see the harm in sending a text saying off to park/friends house etc so you know where your dc are
i generally mention to mb what we are doing that day but as nick said plans chnage, weather may rain/child meant to see was up all night being sick etc
she also knows we do roughly the same thing, ie see a friend/tumbletots/music/coffee am etc
in the summer we all go out for days out, again i dont ask mb/db if we can go - but we did discuss at interview that im a sociable nanny and go out to farms/zooz/beachs/cinema/bowling etc and they said lovely
i think i did ask if there was anywhere that they wouldnt be happy for me to take them
and answer was macdonalds
and they prefer pizza/zizi/brwsters etc
saying that, i also dont see the harm of having a diary and for nanny to write in it what the plans are, but if things chnage due to the above reasons for you not to get annoyed
you either trust your nanny or you dont :)