Parents frequently feel a pea-sized lump or two in the back of the head or neck in their healthy young child. Most often these are normal lymph nodes -- "glands" that we all have in various parts of our bodies. These can get bigger associated with an infection in the same area. For example, you are probably familiar with swollen glands in the neck from a throat infection that do get smaller after the infection is treated.

From what you have described, your daughter's lump is probably just a normal "gland." It has always been there, sounds like it doesn't bother her, it's not sore, and it's not changing or getting bigger. Parents also sometimes think about cancer when these lumps are first felt. In otherwise healthy, growing children, that is extremely rare, so blood tests and biopsies aren't routinely recommended.

I suggest not doing anything special. Watch for any significant changes in size or if it is associated with some local infection. Otherwise, simply ask your pediatrician to check it from time to time with her regular well checks for continued reassurance.