Talking with child about adoption

I have two sons that we adopted out of foster care (at different times, not biologically related). They are 6 and 7. We have a bio son, age 3 and I am about to have our bio daughter. This last pregnancy has brought up lots of questions and feelings from my adopted children. Can anyone give advice on how much to share and how to explain. They've always known they were adopted, have had continued visits with foster families etc. but now they have questions about their birth parents and where they are. I want to be open with them, but also know they are not ready for details about drug exposure, domestic violence, jail, government involvement, courts....