Regarding your lump, most likely you have a scarred lymph node that will not get smaller. If it is not getting any larger it is unlikely to be concerning. It is worth monitoring, however, occasionally to see if there is any change.
With your other symptoms it is a good idea to see your doctor and discuss all your symptoms to see if you need any other work up.
Your daughter may have simple enlarged lymph nodes, or may be fighting off an infection currently. The tonsils are really a variation of lymph nodes and can get bigger with infection.
When one tonsil is larger than the other and the child is sick, an abscess must always be considered.
With her night sweats, etc. testing for lymphoma is reaonable. By statistical odds, lymphoma is less likely than other things; however, because of its severity it warrants evaluation for.
Another consideration would be infections such as valley fever or histoplasmosis (depending on where you live) that can cause essentially all these symptoms.
I would recommend you continue to follow up with your pediatrician to have her properly evaluated.