Engage the child in conversation when he/she’s around but make it natural conversation, not forced conversation.Most teens are cynical and standoffish to new boyfriends so you have to expect hostility initially.The more selfhood, charm, humor and intelligence you convey the more you will seem worthy of attention. You may get only a polite "No thanks" but that is better than dead silence. Here’s what you do: treat the kid like another adult and engage in conversation and bonding the way you would if you were stuck in a house with random guy or girl.
For each item note how you feel: Let her know if you see D/s as a lifestyle or a form of play. The more honestly felt words you present about yourself the better your chances of meeting a compatible Domme.
If her she calls herself Lady Sarah then your email should probably begin civilly and conventionally enough with "Dear Lady Sarah." Addressing her as Mistress implies a relationship with her that you've yet to earn. And if you've created a web page as I suggested above close your email with the URL.