There are a great many books and worksheets for the teaching of music theory available. Many of these are excellent, and well written and researched for the instruction of music theory.
There is one thing that over everything else makes certain worksheets and educational materials more effective in this area. This one thing is an educational concept that good educators in every subject understand, and that whether you’re teaching preschool or very young children right up to adults need to understand.
That one thing is concept isolation. The worst theory books are those that introduce everything in a hap-hazard way, and have a mixture of concepts taught on one page.
The best books take one concept and expand on this through different activities based on that one concept. For example if you are working on understanding scales, don’t introduce a rhythmical element to it until the students are very familiar with what you are trying to teach them.
For example a badly written question asks the student to write a scale in a particular rhythm, or fit it into a set time signature. Then the student is not really learning anything – they are trying to assimilate two concepts at the same time. Leave the learning about time signatures and meter for another worksheet focusing on that content, and teach the scales properly.
A much better way to ask them is to ask them to write a scale and don’t mention any specific rhythmic type of note at all. This way the child will be able to concentrate on writing the scale and won’t be split between concepts.
As music is such a multidimensional language with pitch and rhythm coming together it is obviously impossible to isolate everything, but doing it as much as possible will definitely help the students with their learning of music theory.