Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if a child has:
Severe breathing difficulty, confusion or collapse remains an emergency.
Seek urgent same-day medical advice if a baby younger than 3 months has a temperature of 38°C or higher.
Young babies can become unwell quickly, even when the cause is not COVID-19.
Also seek prompt advice if a child is breathing faster or working harder to breathe, is drinking much less, has fewer wet nappies, repeatedly vomits, has a persistent fever, or is getting worse rather than better.
Contact the child’s medical team early if they have a condition that increases their risk of severe illness.
Seek urgent assessment.
Routine medical follow-up is appropriate when symptoms continue beyond several weeks, interfere with school or activity, or repeatedly return.
The article also says
Persistent fever with stomach pain, vomiting, rash, red eyes, dizziness or unusual sleepiness in the weeks after COVID-19 could suggest MIS-C.
Every sign above is quoted from the sofidex article on Infants coronavirus (COVID-19) in children. If you are not sure, err upward — it is always reasonable to be checked.