Hi. I think there are many reasons to consider, children are very intuitive you have to analize what have changed since she started crying (maybe her parents are expecting a new baby); there must be a reason of why and it is at times hard to figure it out even a simple change in their rooms or your playroom can trigger this kind of melt downs.

Anyway, my only approach to your situation is changing her emotional and mind state when she arrives. Ex. Find out what she likes best some girls love crafts or playdough or tiny toys. Get ready a little basket just for her, as soon as she arrives offer her that special activity just for her. I have a little one that his brain doesn't take transitions easily for him every morning there are special things waiting ready. So, now as soon as he arrives he runs into my home happy and he even forgets to say bye to his dad or mom. He loves his little activities and joins the group when his mind has settled (usually 5 to 10 min.) He also has been going to preschool and he still cries there (they do not know how to settle him down).

When you mentioned that she cries after almost every transition then it means that her brain is having a hard time to switch from one activity to another which results in melt downs, try to give warnings at least five to two minutes before every transition ex. We'll have snacks in five minutes start to finish what you're doing. Etc. Also you can give her a bit of control of choosing cleaning up in two of five minutes.

Also, self regulation or self control doesn't come easy at that age a quiet place for that age must have something soothing ex. Big picture books, little stuffies, fidget toys or something that can distract her mind and help to settle. Due to stress I have also placed a little bucket with warm water dishsoap and a sponge so they can squish it (only for that child, I just explain the others that they will have chance to explore it once our little friend is done)

I know this might disrup your daily routine but, it is only until her brain settles and recognizes eventually that only in your home feels secure and nurtured. Don't give up. I have also tried (and still do) play soothing, calming music.

You also said that it started two months ago (I just noticed close to the beginning of the school year); maybe it is too many transactions and her brain doesn't have enough time to properly settle down.