Usually a Rathke's cyst is a developmental thing, as the pituitary develops off the roof of the mouth in the fetus the remnant can remain as a cyst and this can be seen in children with imaging, I have 2 patients, both have deficiencies due to the cyst mostly due to poorly functioning pituitary glands. The hemihypertrophy can go along with several syndromes, I am glad they are ruling out Wilm's tumor, be sure that they have you with a genetics specialist as well as there are endocrine problems that are also associated with hemihypertrophy and the genetics and endocrine specialists work together.
You sound like she is well monitored.
Could the Rathke's cyst and other structure be putting out growth hormone contributing to her fast growth rate and large size? Do Rathke's ever grow and do they every have to be removed??
Doubtful, usually it is the cause of less gh secreting tissue there and usually gh deficiency, excessive gh is acromegaly and usually due to a pituitary adenoma and can be seen on mri at times if large enough. A blood test for gh and somatomedin C can help look at whether there is excessive gh. My thought is that if the growth is too fast we need to consider early puberty.