However, with this sentence structure you would use the plural forms: On average, kids spend around ten hours a week watching television. I wish I could tell you why.

We would say: " The average kid spends..." I don't have a good explanation for why this is, but it's definitely the correct way.

It should be: The average kid spends around ten hours a week watching television. When you use the word "average" with this sentence structure the subject (kid) and verb (spends) take on singular forms.

thank you :-)

When you add "average" to a noun, you speak about one typical object. There are many objects like this. That's why it is average(=typical, usual). One with characteristics of the group, to which it belongs. You could say "On the average kids spend..." meaning that it is the usual amount of time spent on watching TV (or the mean value with some watching 5 hours a week and some 15).