When the toy cupboard is full
Buy a different kind of thing, not a better version of the same thing.
A full toy cupboard at one end and somebody actively downsizing at the other. Different problems, and one answer that works at both.
Family gift-buying has two hard ends and an easy middle. At one end is a child with several sets of grandparents, a birthday and Christmas four months apart, and a cupboard that has been filling for six years. At the other is somebody who has spent a decade trying to have fewer possessions and does not want another object however nice it is.
Both ends have the same answer, which is that the gift should not be a better version of something they already have. For a child, that means a different kind of thing entirely: something they are in, something involving a specific adult, or the first proper version of a real tool rather than the toy one. For a grandparent it means something with no storage cost: time, a recording, their own history, or something to show visitors.
The middle, teenagers, is its own problem and it is mostly about timing rather than taste. Interests move fast at that age, and the classic wrong gift is not boring but roughly two years out of date. The fix is intelligence gathering from a sibling or a friend rather than from the person, who will say anything is fine and has told somebody else exactly what they want.
One rule runs through all of it: whatever the children make, do not tidy it up. A drawing by a six-year-old is a document of what they were like at six, their handwriting exists for about a year, and every edit an adult makes removes the part that was valuable.
Buy a different kind of thing, not a better version of the same thing.
Recognition of their own bear is instant and total, in a way recognising a drawing of themselves often is not.
Grandparents keep an astonishing amount. The question is what is worth them keeping.
Treated as an actual interest with depth to it, the gift list gets substantially better.

A christening present is a message to somebody who does not exist yet. That is a strange brief and it has a good answer.

Hard to buy for is not one problem. It is four, and they have different answers.

A graduation present has to survive six house moves. That eliminates most of the list.

You are buying decoration for a room nobody has decided about yet. That is a difficult brief.

A teacher leaving after twenty years has received roughly two hundred mugs. Aim slightly higher.

Everyone buys for the baby. Almost nobody buys for the two people who have not slept.

Every friend group is secretly a fighting game roster. The moves, the taunts, the ancient rivalries. Here is how the group chat becomes a hand-painted fighter.