If a child is scared to go to the bathroom himself, may one enter with him?
One should try to calm the child down in other ways, such as to slightly open the bathroom door and the like. If necessary one may escort the child to the bathroom.[1]
[1] In Basra 3:2 Admur writes that if a person is scared one should place his hand on their head through a window, however he may not enter the bathroom. Seemingly there is no difference between a child and an adult as explained in the first Q&A, and hence when possible the child should be calmed in other methods. Nevertheless being that the fear of a child can be dangerous for him, [as brought in 328:15 that if a child is locked in a room one may break it down on Shabbos because he may get scared which can be dangerous] one may certainly be lenient when necessary to enter with the child in order to calm him.