There should be a homework timetable and fixed guidelines for how long they need to spend on each subject. For example, 3 subjects x 30 minutes per night. Even doing prep at school, they should at least come home with one homework to do at home.
Can you hunt through your paperwork and see if you can find the homework policy, or see if it is in the DC's planners?
You should be able to see from their planners what is set each night and ask them to tell you exactly what this entails. I find though, that they tell you they did it in prep and don't have the evidence with them!
What it shouldn't be is simply 'finish work from the lesson', although this can be part of it but not the major part.
If you do all this and conclude that there is a problem, contact the form tutor, who can monitor what is being set and intervene if necessary. You are perfectly right to be concerned if there is either too much or too little homework.