My suggestion would be to adopt the approach I use with hopped cider...
Wait until you have racked to secondary, or if you don't do this wait until the SG is around 1.020 - 1.030. At this stage you should have enough alcohol to extract flavour and also deal with any potential nasties that you introduce.
Then, add the lavender in a "teabag" on a string with something like marbles added to make it sink and steep (jiggle it), then taste each day to get the flavour right. I don't know how lavender behaves but hops give up their flavour quickly and every day of steeping can make quite a difference. Take the bag of lavender out when you are happy with the cider flavour and continue the process as you normally would.
Cheers!