Understanding isolated words is not adequate for the task of understanding language. All languages have rules regarding how words can be combined to form sentences, and an implicit understanding of the rules of sentence structure and phrasing is essential to language comprehension.
It is difficult to assess a child's syntactic knowledge without using printed text, but as was the case with semantics, if the child must process text to take the test, then the test becomes more of a decoding skills test than a test of syntax. It is possible to make some estimations about the child's productive syntactic knowledge by listening to the sentences that the child forms when she is talking, but teachers should be aware that the productive syntax may reflect the child's own dialect, which may follow non-standard syntactic rules, and may not reflect the child's ability to process and understand syntax in other dialects (it is common for people to speak one dialect, but be able to understand many other dialects).
A child can also be asked to identify sentences (spoken) that are syntactically incorrect (e.g. "Jane and her dog the hill they climbed it" or more subtly, "Me and Jane walked up the hill."), and additionally, the child can be asked to restructure the sentence correctly.
Another common syntax test involves presenting the child with sentences which have one word omitted, and asking the child to suggest words that could fill the blanks. In this case, the meaning the word is not what is being evaluated, but instead, the child is graded based upon the syntactic appropriateness of the word. So, for example, the child may fill in the sentence, "Mary fell off of the ___" with any of several obviously appropriate words such as "chair" "house" or "wagon," but credit should be given for any noun that the child supplies (e.g. "leaf" "brain" or "mop") because they are all syntactically correct.
Another assessment of syntax involves the child's ability to combine simple sentences into complex sentences, and to add modifiers appropriately. So for example, "Brownies taste good" could be combined with, "Mary likes to eat brownies" to make the complex sentence, "Mary likes to eat brownies because they taste good." Similarly, children could be asked to appropriately insert the words, "brown" "big" and "quickly" into the sentence, "The spider ran up the wall" to make a more complex sentence such as "The big, brown spider quickly ran up the wall."
Similarly, a child's syntax can be assessed through a test of her ability to change tense and modifiers of sentences. For example, a child could be asked to restructure, "I went to the store" to the future tense.