Moving with 18 Month Old and Almost 5 Year Old.

Duplicate her room now in the new house. I would even go so far as to say if the baby has his/her things in the older ones room, move them and put them in the new room for a few months until things settle down and YOU have the time an energy to deal with any upsettedness.
and keep as much of the rest of the routine the same as you can.
I found that it took about a month for me to adjust to our new house. My kids seemed ok in about that amount of time as well.

Since your daughter is already familiar with the new house, you may want to take her over that a few times before you move, to show her where her new room will be. Explain to her that she is a big now and she needs her own space, with her own toys, etc. Try to make it about her, this way she'll feel as tho she gaining rather than loosing or missing something. I hope this helps. :)

Moving is a very abstract concept to her right now. You need to make it as concrete as possible and give her any decision-making control tha tyou can.. Take a picture of her new room or draw a diagram and ask her to draw where she will want to put her bed and dresser, etc. Have her pack and take our her boxes sooner than later. have her write a goodbye note to the house and telling it to take good care of its new family. Give her a couple choices of paint chips for her room and let her pick the final one. Maybe she could buy flowers for the house for the new owners.

Finally, make sure you talk about being sad and excited and let her know it is okay to feel that way, but make clear things like she won't be able to just walk into her old house because it won't be her's. Let her take some pictures or take pictures of her in each room and let her make a little scrapbook with construction paper and stickers.

Congrats/good luck.

Since you are moving across the street, hopefully it won't be too hard on her.

Take pictures of her current room. Paint and organize the new room as close to the same as possible.

Since you are friends with the current owners, ask if you can come visit and stay for a bit in what will be her room.

M.