If it's hard, it's very likely a lymph node that will require a biopsy. Normal lymph nodes are soft and palpable.
As a lymphoma survivor (Hodgkins lymphoma diagnosed 6/2008), I'd highly recommend following your instincts and being diligent.
It's possible it's nothing - 95% of lymph node enlargements are harmless, 5% are concerning. There were no symptoms in my case, just a few small, lymph nodes in my collar bone area which are never a good sign. All other bloodwork, etc. was normal.
Does your daughter have any other symptoms: nausea, fever, night sweats, weight loss? If so, I'd mention them to the pediatrician ASAP.
If you do biopsy, do not get a fine needle aspiration. They're much less accurate than a complete biopsy which isn't a whole lot of fun.
If you want more information on my experiences and my knowledge about the lymphatic system, I'll be happy to share it.
MY BEST ADVICE: Stay off the internet! I pretty much had already diagnosed myself with cancer before I actually was which gave me 4 weeks of extreme anxiety.
Good luck! Hoping it's her body's means of dealing with a rather stubborn infection.