I am African and i want to tell you the person that advised you to get them in with a visitor visa gave you the worse advice ever. Any African family specially a woman+kids (unless they are from a diplomate family), specially when school is in session will most likely get denied entry.
I went to the green card process while family was outside, it took a while but the wait for worth it. They travel with a peace of mind. B1/b2 visa is never guaranteed you will get in.