The lump in the neck may be responsible for neck pain
.It may be a cyst, lymph mode, abscess
ar anything and should be examined by doctor/paediatrician.
You should have the blood counts, physical examination, and sonography
and if required fnac of the lump to know the cause of such swelling.
Meanwhile you may give her pain killers like motrin, advil to control the symptoms till you visit the doctor.
The oral antibiotics
course may be required if it is due to some infectious etiology.