Fewer materials, better ones, and a shelf they can reach.

A cupboard stuffed with novelty kits produces less work than one good box that is always available.

The instinct when a child shows interest is to buy more. More colours, more kits, more sets with themed contents and instructions on the lid. The effect is usually the opposite of the one intended. A large assortment invites sampling rather than working: the child opens everything, tries each thing for ninety seconds, and never settles. A small set that behaves well invites depth. If there are six colours rather than sixty, the child mixes, and mixing is where colour is actually learned. If there is one good brush rather than a bundle of splayed ones, they find out what that brush can do.

Quality matters more than range because bad materials misbehave in ways children read as personal failure. Thin paper that tears under a wet brush, hard colour that will not lift, a pencil that will not make a dark mark: the child does not conclude that the tool is poor, they conclude that they cannot do it. Buy less and buy things that work, and check that whatever you are offering is suitable for the child using it. That is a practical judgement to make once, at the shelf, rather than a running commentary during the session.

Then make it reachable. Materials that live on a high shelf and come down as an event happen rarely and carry expectations. Materials that live at child height in a shallow box, with paper in the same place, get used on ordinary afternoons for ten minutes at a time without anyone announcing an activity. That ordinariness is the goal. Art that requires an adult to set it up, supervise it and pack it away becomes something the adult must be in the mood for, and a child's access then depends entirely on adult energy, which is a poor foundation for a habit.