Sometimes it can be hard to decipher what your child is drawing or creating, even when your child knows exactly what it is. Try asking open-ended questions like "Tell me about your picture".
You can also describe specific actions your child is doing by sayings things like "You're making long lines, I see you are using pink, purple, and yellow." Try describing how your child is using the materials and tools by saying things like, "You are using a rubber stamp", "I noticed you are making small dots" or, "You are using two crayons at the same time!"
Focus on the process, not the product – Encouraging your child in the action of unstructured art helps them build their own motivation to be creative. Part of focusing on the process involves encouraging effort. Letting a child know that you noticed how carefully they were working or how long they spent on a project will go a long way!