Most children do not know how to build tension, their stories tend to jump from one point to the next or they tend to solve the problem too quickly and as a result the stories are not gripping.

Example:

I heard a creaking sound in the house. I wandered
downstairs and saw, to my horror, a masked man. He must be a burglar! I stood rooted to the ground, I was shaking with fear. The burglar suddenly turned and saw me. He advanced towards me and then he caught me and gagged me.

For some children who read a lot and can pick up the grammar subconsciously, grammar will seem easy for them. However for many children, while they may be able to understand the easier grammar items, they have difficulty in understanding the more complex ones.

Here is a short explanation on the difference between the past perfect tense and past tense.The past perfect tense is used mainly when there are 2 actions and one action happened before another action.

Here are some of the suggestions:

1) Magic Treehouse series

This is a very common series that talks about 2 children who go back in time to interesting events that happen in the past. For example: the volcanic eruption in Pompeii and how they try to solve some puzzle there or help someone during their travels.

I found these authors good for upper primary kids:

1) Sophie McKenzie - Blood Ties

It has won numerous awards and the plot is thought-provoking and gripping. It is about cloning and people's reactions to it. It links 2 kids who are actually clones but have no idea that they are clones and they need to figure out who their parents really are and how they can escape the clutches of a terrorist group.