Start with the reassuring part

The Royal Children's Hospital Melbourne opens its guidance with the base rate: "it is common for children to hit their head. In most cases, this just causes a minor head injury, resulting in a bruise or lump." The NHS agrees that "most head injuries are not serious". Toddlers fall constantly, and the great majority of those falls end in a scream, a cuddle and an egg-shaped lump.

The reason to know the thresholds anyway is that the ones that are not minor tend to declare themselves in the first hours, and knowing what you are watching for makes those hours calmer.

Call 999 straight away if

The NHS lists these after someone has hit their head:

they have been knocked out and have not woken up;
they cannot stay awake or keep their eyes open;
they have a fit (seizure);
they have fallen from a height of more than 1 metre or 5 stairs;
they have problems with their vision or hearing;
they have a black eye but did not hit their eye;
there is clear fluid coming from the ears or nose;
they are bleeding from their ears, or have bruising behind their ears;
there is new numbness or weakness in any part of the body;
there are problems with walking, balance, understanding, speaking or writing;
they hit their head at high speed, such as in a road accident;
there is a head wound with something inside it, or a dent in the head;
there is a bruise, swelling or large cut on the head and the child is under 1 year old;
their behaviour has changed — more irritable or distracted, or losing interest in things around them.

That last one is the one parents most often talk themselves out of, and it is the one that matters most in a child too young to describe a headache. RCH sets a matching rule for Australia: call an ambulance immediately if a child injured their head at high speed or from a height above one metre, loses consciousness, keeps vomiting, or has severe symptoms that do not get better — "even if they do not have concussion symptoms."

Call 111 if

The NHS lists vomiting, dizziness, a bleeding condition or blood-thinning medicine, and "any other symptoms you're worried about" as reasons to get help from NHS 111. It adds a specific instruction for this age group: call 111 rather than using the online service if you need advice for a child under 5. If you are told to go to A&E, do not drive yourself there after hitting your head.

The AAP's list of signs of a more serious head injury is a useful companion, because it is described in terms of what you would actually notice: a constant headache, particularly one that gets worse; slurred speech or confusion; dizziness that does not go away or keeps happening; extreme irritability or other abnormal behaviour; vomiting more than two or three times; stumbling or difficulty walking; blood or watery fluid from the nose or ears; difficulty waking up or excessive sleepiness; unequal pupil size; double or blurry vision; unusual paleness lasting more than an hour; seizures; difficulty recognising familiar people; weakness of arms or legs; and persistent ringing in the ears.

The AAP also flags that its general advice is not written for children under 2, or for children with neck injuries, existing nervous system problems, developmental differences, bleeding disorders or easy bruising — those children can have more serious problems after a mild head injury, so a lower threshold applies.

Looking after a minor bump at home

If your child cried, settled within a few minutes, and is behaving like themselves, the NHS approach is straightforward.

Cold on the lump. Hold an ice pack or a bag of frozen peas wrapped in a tea towel to the area regularly for short periods over the first few days to bring down swelling. The AAP suggests around 20 minutes of a cold compress.
Let them rest, and let them sleep. The NHS says explicitly that "you or your child do not need to stay awake if you're tired." The old advice about keeping a child awake all night has gone.
Stay with them. Make sure an adult stays with the child for at least the first 24 hours. The AAP says the same and adds that a child may need watching carefully for a few days, because signs of a more serious injury can be delayed.
Pain relief. The NHS allows paracetamol for headache, using a product intended for children and following the label.
No rough play. The NHS says children should avoid rough play for a few days, and that contact sports should wait at least 3 weeks.

RCH advises watching a child carefully for 24 to 48 hours after any head injury and speaking with a doctor if new signs develop or you are worried.

Concussion, in plain terms

RCH describes concussion as a type of mild head injury that temporarily changes how the brain works and does not show up on a CT scan. Symptoms can be physical (headache, nausea, dizziness, fatigue, poor balance, sensitivity to light or noise), emotional (anxiety, irritability, confusion, sadness), sleep-related, or cognitive. Children with concussion "usually start getting better within a few days and fully recover within two weeks, but it can take longer for some children." The NHS says symptoms can last up to two weeks, and to see a GP if they last longer than that.

NICE guideline NG232 sets out how head injuries are assessed and managed in hospital, including when scans are needed — which is why the decision about imaging belongs to a clinician who has examined your child, not to a symptom list.

Preventing the worst falls

The AAP's safety advice for two- to four-year-olds is specific about where serious falls come from: play equipment, windows, stairs, bikes and anything climbable. It recommends gates on stairways, operable window guards above the first floor, fencing the play area, and soft surfacing under play equipment. Its blunt summary is worth keeping in mind: at this age a child "doesn't understand dangers or remember no while playing and exploring."

Toddler Fell and Hit Their Head: What to Watch For