My son has this as well - on both sides of his neck. DO NOT PANIC...
They come and go. We take him in to the Dr when they get bigger - tests are ran - and they still come back clean - it's normal...not every child gets these enlarged glands, but it does happen.
DO NOT FREAK OUT!!! DO NOT PANIC!!! Just look after it - keep a log or a notebook - not EVERY day - maybe once a week so you can see differences in it - but DO NOT BE A HYPOCHONDRIAC about it - it will make your daughter self-conscious and just end up flipping her out as well.
You need to have a good talk with the doctor WITHOUT your daughter in the room so you can express your concerns......have the doctor show you charts, etc. and get on the same page with them...but they HAVE to hear your concerns and not push you off...
The problem with technology today - people who don't attend medical school have access to information and without all the information - form diagnosis' without knowing all of the stuff....so while you've read things that tell you it could be turn into a tumor - stop and do yourself a favor....trust your pediatrician - if you don't - then find another one...or ask for a second opinion...but you really need to calm down...i'm NOT trying to be mean or condescending, however, each of us has cancer cells in our bodies - yes, we do - so you are freaking out over something that COULD be....just keep an eye on it. don't measure it every day. you'll turn your daughter into a paranoid hypochondriac as well....(not saying you are) but really, trust in your pediatrician.