Snape's mind was made up. The child was a doppelganger; he had to be secured. Preferably brought into the castle, or if not then to a Fideliused safe-house. Spinner's End would be ideal.
If he could get her away as well, then so much the better. It's not as though I want to be the one to raise the brat. he told himself firmly.
"I'll be in contact soon," he said by way of fond farewell. "Pack only the essentials."