If I had to guess, I'd say a pre-sitting house call would be a visit to the house where the sitter is going to work before that actual sitting job begins. The purpose of the visit would be to familiarize the sitter with the child or children to be taken care of and find out if there are any special circumstances. This would be done probably for a long-term sitting job - more than one day.

A house call is a professional visit, as by a doctor or sales representative, to the home of a patient or customer. I've never heard of a house call in the context you gave, but it suggests the would be baby sitter is making a house call to deliver a professional service.

Hi! Well... I've never heard of the word "housecall". The word that comes to my mind is "household" though and, that could make sense. Householding is like housekeeping, which means helping with the chores (cleaning, cooking etc.) Hope that helped :)