What to say to a child when he says he still got a bad dream even after reading adhkar before going to bed

Tell him the dream is from the shaytan and that it cannot harm him, and teach him what the Prophet ﷺ taught: to seek refuge in Allah from the shaytan and from the evil of what he saw, to blow lightly to his left three times, to turn onto his other side, and not to tell anyone what he saw (Muslim). The adhkar are a means of protection, not a promise that he will never see something he dislikes.