The task was introduced to the child using three practice items that followed the exact same format as the test items
The first step is to remove the passive voice – “the task was…”. “The instructor introduced the practice items to the child, using the same format as the test items.” Then, to remove the ambiguity of “using,” build a subordinate clause that separates “which” from “who” -- …”introduced the practice items, which used the same format as the test items, …” So the final rewritten sentence is: The instructor introduced the practice items, which used the same format as the test items, to the child. You could also get a more active structure by: “The instructor introduced the child to the practice items…” a more logical reversal but a slightly invasive change.
The child first saw the topic through the three practice objects that were identical to the test object's format.