Yes,the answer to the question is what will get you to the point .Its not difficulty in the child its the time of childhood, if not taken care will pass on to teens to rest of the life.
When they dont get it perhaps there are areas that needs to be reconsidered in your teaching. Are you going right , they all are different and the ways to get the same topic to each is different ,but most of us dont have the time to get into such details.
Every child has a different way of getting on the topic.Some of them want you going around the bush to get to the topic , Some want you to keep it short in both words and content, Some cant take it if it doesnt come logical, Some want you to use the vocabulary only they are familiar with and Some wont get to the topic till its 5th or 6th time.
Its time to stop coining or terming the child difficult in anyway, with easy availability of resources and media for education in every form . May be what’s lacking is the human touch of able to give the topic a meaning rather than just presenting it.
When a child hasn’t got the concept even after the topic has been completed from your side, teaching needs to take the responsibility and find changes in content and parts missed or not addressed from a Childs point of view.