Children and TV. Your thoughts?

I'm a nanny of a 3yo and 5yo and they LOVE TV. They always ask to watch shows. Then another. Then another. I watch them 8hours a day and only allow them to watch 1 20 minute show in that time (if that some days I don't play any shows for them) 
Every kids show teaches something different whether it be education (team umizoomi) or life lessons (Daniel tiger) but these 2 kids I watch retain NOTHING AT ALL. They watch the episode and when I ask them about it they have no recollection. The only show they seem to retain info on is Wild Kratts. 
So I told them they can only watch that if I put on a show for them cause it's the only time they'll tell me about it and prove they learned something. 
What do you do for your little ones and do you think this is a good method? 
Please no hate in the comments this is a judgement free zone!!! ❤️❤️

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