Persistently enlarged lymph nodes (glands) cause terrible anxiety in parents. They are most commonly caused by infections. They are usually limited to the area they drain, for example in the neck after infections of throat/ear/scalp etc. Sometimes it can take weeks or months for them to shrink in size and that can be worrying for parents and carers.
Most often, parents only need reassurance but if there are features like fever, night sweats, weight loss etc. or if there is generalised enlargement in multiple places, then, blood tests can be done to rule out rare but serious conditions like Leukaemia and Lymphoma etc.

https://www.nhs.uk/conditions/swollen-glands/