A., you sound so much like me. When I found out I was having my third and fourth child, my mother-in-law said it was because my sister-in-law was pregnant, and that is the reason why I got pregnant, and even though we are adults, we want to try to please every one. But we can't. I would wait to make sure to tell any one else. Every one will think your crazy, but that is your decision. I think you will be able to handle being pregnant. Look how much you have accomplished so far, with two boys in tow, and a babysitting job, on top of that. THAT should tell every one that you can do it.
I would let any new people, after they accept you as their babysitter, this way, when you have to take off once a week, they can have plenty of notice on why you are taking the day or part of the day off. This way they are not shocked on why you have to take the day off.
My best friend is a babysitter, and when she started when she was pregnant, she told her parents right away, this way they knew they were expecting once a month, at first, then once a week further along, that she was taking off.
Hope this helps.
P.S. I also married my high school sweetheart. Been together for 16 1/2 years, and been married for almost 12 years. 12 on Monday, May 12.