Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services if a child:
Has severe difficulty breathing, is gasping, or cannot speak or feed
Develops blue, gray, or very pale lips or skin
Collapses, becomes unresponsive, or is very difficult to wake
Has signs of poor circulation, such as cold mottled skin with confusion
Has sudden severe chest pain, especially with fainting or breathing trouble
These are important but are not always emergencies.
And for anyone, someone who:
has chest pain, tightness or pressure lasting more than a few minutes
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek urgent same-day medical advice for noticeably faster or harder breathing, new swelling, repeated vomiting, much poorer feeding, fewer wet diapers, or a sudden drop in energy.
Rapid weight gain over a short period may represent fluid buildup if the child's care team has advised weight monitoring.
And for anyone, someone who:
is getting rapidly more short of breath, or has gained several pounds in a few days with swelling
3
Does any of this apply?
Book an appointment
Continue routine follow-up even when the child feels well.
Heart function or treatment needs can change before obvious symptoms appear.
The article also says
Contact the child's cardiology or primary care team soon for gradual changes, such as increasing tiredness, slower growth, more difficulty with usual activity, or possible medicine side effects.
Every sign above is quoted from the sofidex article on Heart failure in children. If you are not sure, err upward — it is always reasonable to be checked.