Here is an article about questions we can explore with our children (at any age, really). Using the child’s mother tongue will ‘push’ / ‘pull’ the child to think new things in her/his mother tongue that will benefit both the mother tongue and her/his cognitive growth.
Do not make this a test – see it as an opportunity to explore ideas with your child. If the timing doesn’t seem right, leave it for another time. I’ve found that travelling together in a car or some other form of transport can provide opportunities for this. Enjoy the conversation that has viewpoints but no concrete answers!