I was fixing some toys of my 5 year old and he asked me if he could watch the glue dry... I told him "son, that's like watching grass grow π"... his reaction, "really π±... can I go in the backyard?", me "yeah, sure, I guess π€¦ π".
He's still there... sitting on the ground, watching... asking every 2 minutes if the grass is any taller π€£.
Go out every day with a ruler or a tape measure and take a picture with a measurement. He might not notice or remember the daily growth but after a a few weeks there should be a noticable difference if the climate of your area is still warm enough.
I loved the plant unit in elementary school. We had a planter with glass on the side, and for the first week or so everyone would rush to check how the seeds were doing. Then we got bored until the first seed sprouted.