For a beginner, s/he has difficulty to see check and capture, let alone threat. If s/he is not rushing, s/he would try to scan the board, from right to left, from top to bottom, and repeatedly. If s/he is patient, s/he will check every square (64 of them), every vertical line (8), every horizontal line (8), every diagonal (16). Even with such complete scan, s/he sometimes still can't see. The reason is that s/he can't connect information together and can't prioritize data. S/he wastes a lot of time on useless areas (80% of the previous scan) but skimps on the key areas. Sometimes his/her vision is so tunneled that s/he just can't see the one-move checkmate no matter how long s/he scans the board.
There is no quick way to improve his/her vision. No logical reason will help him/her understand the problem and find solution. The only possible approach is to do a lot of exercises, such as one-move checkmate. A lot, I really mean a lot, not 10 or 20, at least hundreds or thousands. They are simple and they will not help your child much with chess knowledge or strategies. But they will enable him/her see things. S/he should do the exercises in a fast-paced batches, such as 100 problems in half hour. Do them often until s/he don't miss the checkmate or capture anymore.
Threat is more difficult to see than check and capture. Many times it will take at least 2 to 3 moves. This will be a training topic in next step.