Brain tumors are rare but yes, important to monitor, I would first address the early puberty, have her referred asap to peds endo, she may need labs done and assessment to see if her brain is signalling too early, most of my patients have unknown reason and the few that need treatment do very well. Labs to get include LH, FSH, free T4, TSH, estradiol. If she has suspicious exam or screening labs a leuprolide stim test is needed, baseline and 1 hour labs after giving leuprolide which in a nonpubertal child will show no change an in a child with early pubertal signalling will show a peak LH of 6 or 7 and labs done 24 hours later also will show a rise in estradiol. A pelvic ultrasound is also important, she may have a simple cyst on her ovary causing estrogen to stimulate her breast tissue, this is benign usually and simply can be monitored as long as things do not progress.
Start with referral to peds endo and go from there.
I am experiencing the exact same thing! My daughter just turned 6 in May and has developed small breasts as we as hair under arms (no pubic hair though), and a fowel smell below. We have an appointment this Saturday morning.....I am nervous too :0(