tehillahdreams.com

I dreamt that I was pushing my niece Tianna in a stroller. She was a baby. We got on the elevator and got off on the 6th floor. After we got off the elevator, I looked up and the baby was gone. I was looking everywhere for her and was frantic. Suddenly my Mom appeared in the dream.(She's deceased IRL) to let me know that the baby was fine, and that my Stepfather had taken her to clean her up. I was relieved. In real life, my niece is 25 years old, and not a baby.

This is an interesting dream...

The name Tianna means follower of Christ. Additionally,

Where did the name Tianna come from?
Tianna is an alternate spelling of Tiana which is short for Tatiana. Tatiana is a name of early Russian-Christian origin; it was bestowed on baby girls in honor of a 3rd century saint who was martyred in Rome for her Christian beliefs under the pagan Emperor Alexander Severus.

You both got off at the sixth floor:

The number six refers to many things in the Bible, but it's first mentioned in Genesis 1:31 when God created man on the sixth day. According to Bible scholars, just as the number 7 typically signifies completion or perfection especially of God, the number 6 is one shy of that, which means it signifies imperfection.

It is noteworthy that your Stepfather took her to "clean her up" when she was on the sixth floor (imperfection).
I am puzzled as to why it was your "stepfather" and not your "father"... was your stepfather the father figure in your life?