Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek urgent or emergency help if the child:
Use local emergency services or go to the nearest emergency department when there is immediate danger.
Contact the child's treatment team promptly if symptoms suddenly worsen, medicine causes concerning changes, or the child develops marked agitation, unusual behaviour, hopelessness, or thoughts about death.
Arrange a routine appointment if worry has lasted several weeks, keeps returning, or affects sleep, school, friendships, physical comfort, or family life.
Help is also appropriate if a child increasingly avoids activities, repeatedly seeks reassurance, or has frequent unexplained headaches or stomach aches.
The article also says
You do not need to wait six months to seek support.
That period is part of formal diagnostic criteria, not a requirement for receiving help.
Do not assume serious physical symptoms are caused by anxiety.
Every sign above is quoted from the sofidex article on Generalized anxiety disorder in children. If you are not sure, err upward — it is always reasonable to be checked.