السلام عليكم ورحمة الله وبركاته Ustadh, what do we believe about children who die? Do they all go to Jannah, or is this only for Muslim children?

This is a saying that all of them enter jannah as ibn abd albar has reported that it is the saying of the majority. And another saying is that they are justly tested on the day of judgement. And Allah knows best and He is the Most Just.