The short answer
Reading leads. Quran reading is a skill that takes years and gets harder to start the longer it is left, while Islamic studies can begin at any age and is absorbed in small pieces. Run reading as the lesson and Islamic studies as the last few minutes of it.
Why reading leads
Learning to decode a script is the one part of this that is genuinely age-sensitive. A child who starts Arabic letters at six finds it ordinary. The same child starting at fourteen finds it embarrassing, and embarrassment is a far bigger obstacle than difficulty.
The Kalimas, wudu, the stories of the Prophets -- none of those get harder with age. They can be picked up at nine as easily as at five, in a car, at bedtime, in ten minutes at the end of a lesson.
The case for not separating them
- A second lesson is a second thing to drop
- Families who book Quran and Islamic studies as two separate classes usually keep one and lose the other within a few months. The one they lose is almost always Islamic studies.
- Attention is the real budget
- A child has a limited amount of sitting-still in them. Spending it on two lessons rather than one does not buy twice the learning.
- The last five minutes are free
- At the end of a Quran lesson a child is already settled and already with the teacher. A dua, a Kalima, a step of wudu, a story -- it fits, and it costs nothing extra.
When Islamic studies should lead instead
- A teenager who has never been taught either. Start with meaning, because being handed a Qaida at fifteen rarely survives the first week.
- A child who is refusing the reading outright. Build the interest first and come back to the letters.
- A child who already reads fluently. Then reading is maintenance and meaning becomes the main work.
- Ramadan, or any point where a child is asking the questions themselves. Answer them while they are asking.
Related
Islamic studies for kids, age by age
What belongs at each age.
Islamic studies for kids: where to start
The first month.
Islamic classes for kids
How we teach it.