I know the feeling. Something has happened, a fall that needed a trip to A&E, a complaint from a parent, a change in who lives in your house, and underneath the immediate worry about the child or the situation, there is a second worry: do I need to tell my regulator about this, and if so, how fast?
Most of us learn the answer the first time it happens, usually under pressure, usually by phoning someone and asking. It is worth knowing the shape of it before that day arrives.
There is no single UK-wide rule
There is not one general rule you can apply everywhere, and that is the trap. The categories, thresholds and deadlines are set out nation by nation, and they are more specific than a general seriousness test.
England requires certain significant events to be reported as soon as possible and within 14 days, but explicitly excludes minor injuries, routine hospital appointments and safeguarding referrals about a child. Scotland runs shorter clocks for several categories: reportable accidents within 24 hours, and outbreaks, deaths or allegations reported immediately.
The categories broadly cover things like serious accidents or injuries, allegations made against you or anyone connected with your setting, a change in who lives or works in your household, a serious illness outbreak, and safeguarding concerns involving your service, but the exact definitions and deadlines are set by your own regulator, not by a general instinct about how serious something feels.
Find your regulator's actual list
Your regulator, whether that is the Care Inspectorate, Ofsted, CIW, or your HSC Trust, will have a published list of notifiable events and a form or process for reporting them. If you have never actually read that list for your own nation, it is worth ten minutes now, while nothing is on fire, rather than the first time you need it.
Habits worth building now
Keep your regulator's notification contact details somewhere you can find them without searching, not buried in an old email. Write things down as they happen, not from memory afterwards, because the record you keep at the time is the one that holds up later.
And if you are ever unsure whether something meets the bar, ask, though remember a call for advice does not replace the formal notification itself. Most regulators require that in writing or through an online form, Ofsted's online incident reporting route in England for example, so make sure the official notification still goes in even after you have had that conversation.
This is general guidance, not a substitute for your own regulator's current notification requirements. Check directly with the Care Inspectorate, Ofsted, CIW or your HSC Trust for the exact list and timescales that apply to you.