I want to send an obscene postcard to my young daughter.
That may be a bad idea. Be careful to think about the consequences before doing so.
Because she made me send a postcard to my friends telling them I was taking a trip
I wouldn't go through with this. Remember that you are a parent, and should try your best to be mature and a role model.
It's fine, she'll probably find it amusing. She has a sense of humor.
Even with her sense of humor, she might think that her parent is disgusting for sending her something obscene. And it could create some misunderstanding if it gets into the wrong hands.