Having had taught for many moons, specifically Early Childhood, I’ll tell you simply “You’re the boss!” Maybe let them earn Internet time. Teachers Stores have lil incentive charts. You can give a sticker for good behavior (be specific). Kids want limits because they know they are too young to know as well as you. Just guidance. They can even earn time online with you. They will covet that time and really appreciate it.
Also if they are involved in a specific activity, like an educational game or a story, you want to avoid cutting off their concentration. That is just something to keep in mind.
They are too young to even have a very good concept of time, how much time has passed, how much is left. Get yourself a timer, the kind you wind. That will help them grasp time a little bit. They can get an idea of how much time is left. You can always make it a positive experience, not a negative one at all.
If you want them to be a little more independent, talk to them when you are setting up rules.Ask for their thoughts. Any limits will be much more reasonable to them if they have a say-so and are consulted.
Depending on the ages and your children specifically, I’d say half hour blocks is plenty of time.
Kids that age learn from singing, dancing, listening to stories, playing games, jumping rope, putting on plays, artwork, nature walks, field trips, library visits, dolls, dress up, legos, reading and telling stories. This is all stuff that SHOULD be taught in kindergarten and no longer is. So you best make up for it. (They will learn to read. I don’t understand the big hurry.)
All those things are just as much fun as getting on the Internet.
I’d avoid giving them that much responsibility because it could get out of hand. And as I indicated they are too little and basically will be clueless about setting limits.
Good luck! They’ll be fine. Kids around that age are marvelous and full of wonder about the world.