Describe what you see rather than deliver a verdict.

Praise sounds supportive and quietly shifts the child's attention onto your opinion.

Beautiful, brilliant, clever, the best one yet. These land well for a second and cost something over time. Praise is a judgement, and judgements imply a scale, and a child who learns there is a scale starts working to climb it. They begin producing what earned the last verdict. They start looking up after every few marks to check your face. They become reluctant to attempt anything that might not come out well. The absorption that made the activity valuable is replaced by an eye on the audience, and that swap is difficult to undo.

Description does the same social work without the scale. You have used every colour in the box. That took you a long time. You started in the corner this time instead of the middle. There are three of them and they are all different. You went over that part again and again. Each of these says I watched you, which is what praise is usually trying to say and mostly fails to. It also gives the child words for what they did, which is how they become able to make choices on purpose rather than by accident.

Two things to keep out of it. Do not turn description into a quiz with a hidden right answer, and do not comment on what the picture might mean about the child. Drawings are not windows into a child's inner state, and interpreting them is not something an adult should be attempting, however tempting a dark scribble or a repeated subject may be. Stay on the surface, where the honest information is: what was done, how it was done, how long it took, and what was different from last time. That is plenty, and it is true.