Conflicted..help!

I am in need of some advice. I am currently a full time nanny with a daughter of my own and my second due in September. I am ready for a career change, after being a nanny for the last 7.5 years (4.5 with current family). I would like to get a job in what I got a degree in, business administration. This is where I am conflicted, I am actively searching for a job and just applied for my first one; I won’t hear back for at least 3-4 weeks. Do I tell my nanny family that I am currently looking for a new job or do I wait until I get a job offer? I don’t want to leave them without childcare (I would give at least 2-3 weeks notice), but I don’t want to tell them too soon and be left without a job if they were to replace me (which is possible).