I don't have direct experience in this so perhaps other can give better advice but my first thought is to write up the contract ASAP (not rushed and sloppy though) and have the families sign soon. That way, should any of them decide they are not happy with your terms you are not stuck interviewing to fill a space when notably pregnant. If some decide to leave it might be better they go now so you can fill the space and transition the children before being majorly pregnant or having a newborn with you!

I'd assume your contract won't differ greatly from the agencies so it shouldn't be a problem...but still worth doing now rather than at the very end of your pregnancy!