Learning back sounds for a child can be hard, but teaching back sounds can be just as hard when you can't see what the tongue is doing back there, The palatometer display will show you and the child exactly where the tongue contact is.
In this first clip, notice that the child's tongue tip is touching the front and not in the back like it should in the word chicken.
In this second clip, the same child is trying to not touch the front part of his mouth but by saying it too far back. Notice there is no display (dots lighting up) contact because the child is saying the /k/ too far back in the glottal area. This is often a common error on the therapist part to keep this incorrect placement going without realizing it because it's so exciting to get a resembling /k/.
In this third clip, you and the child will see the back contact forming, like it should. It's not perfect yet but it's where it needs to be.
in this fourth clip, you and the child will see the contact for both /k/ words, bike and key.
this is a good facilitative technique to pair words when they have same tongue contact in both words but are in different locations in words; e.g. final vs initial position of words.