Children's sleep divides into two halves that need keeping apart, because the advice for one is wrong for the other.
For a baby under a year, where and how they sleep is a safety question with decades of research behind it, and the guidance changed completely within living memory. A great deal of older writing — including a good deal still circulating — says the opposite of what is now known. That part is not a matter of family preference.
After the first year it mostly is. Bedtime resistance, fear of the dark, night waking and who sleeps in which bed are questions of what works in a particular household, and the confident claims made on either side are weaker than they sound.
Three things worth knowing before anything else
Broken sleep is not a fault in the first months. Newborns wake repeatedly because they are built to, and most of what gets described as a baby's sleep problem is normal infant sleep. In the early weeks the problem to be solved is usually the parents' sleep, not the baby's.
An overtired child is harder to settle, not easier. Pushing bedtime later to make a child sleepier reliably backfires, and being short of sleep is also the commonest trigger for night terrors — so an earlier bedtime is often the whole treatment.
What matters is not what wakes a child but what they need in order to fall asleep. Everyone surfaces briefly several times a night. A child who fell asleep with a parent on the bed wakes to find the situation changed, and calls out. A soft toy will still be there at two in the morning; a parent will not.
A child's night-time fear is not a mistaken belief to be corrected. It is a feeling to be sat with until it passes.
Advice to be wary of
Three things recur in older material on children's sleep and all three are wrong. That sharing a bed protects a baby against cot death — it does not. That a monitor will let a parent wake a baby who has stopped breathing — a baby who is not breathing needs resuscitation and an ambulance. And that a frightened child can be talked out of the fear with a spray bottle of coloured water, which confirms the premise and entrenches it.
Melatonin is a prescription-only medicine in the UK, started by specialist teams for a small group of children, and it has no part in any of this. Material that treats it as something to buy is describing another country.
The articles below give the current UK guidance, name what the older advice got wrong, and say plainly which questions have an answer and which are simply a choice.