Once again, I asked people for their feedback. This time, I asked my course mates since I had the feeling that the children could not really help me when it came to the heading. As long as there was one, they did not really care about how it looked like. They were more interested in the illustrations.
My course mates on the other hand gave me really useful feedback. One of them asked me if I had tried other versions of the heading and that he could imagine it would look nice if the colours were reverted. I tried that but I struggled when it came to removing the glow. It was an effect that I had added ages before and somehow the version of Illustrator on my laptop did not allow me to get rid of it. So I deleted the whole frame and redid it.
After I changed it, I was not too happy about the way it looked like. Because the heading was really bright, it distracted from the actual content of the poster. Also, I preferred the white font colour. I changed it back to white and deleted the filling (I just left a white frame). I did it in two different ways, so I could go back to my course mates and ask for their opinion.
Note: By now, I have figured out how to remove the glow. Somehow the effect turned into its own illustration and I could just select and delete it. (Photo below)