Just have each child, have their OWN time, at the computer.
You just tell them.
When/if my kids are on the computer, whether it be for doing academics or play, I simply tell them "It is your brother's/sister's turn. Don't interfere. You go and do something else." Each person has their own time on it.
Or since your kids can read by now, simply make a poster, with a "schedule" on it, per what time they can be on it
Computer time, is either, (at least for my kids), doing academics or just playing games. And, I have them do school work/academics on it too.
Not only games and not only them, deciding.
By the way: www.khanacademy.org is a GREAT site, for kids to learn math or to keep up with learning math.
It is FREE.
Even Bill Gates is a fan of this site.
They can do this, during the summer.
During the summer, I highly recommend, having a "schedule" for them, on the computer. Doing academics/studying. Or get workbooks for them.
So that, they don't forget what they learned all year, or to help them increase their skills for subjects that they may be weak in.
Also, when or if you want the tv or electronics off. Just turn it off. Tell them. You decide.
And, have other activities for them to do, for days you are all home.
ie: craft kits, housework, teaching them how to cook, gardening or do container gardening, playing games, etc.
AND, have them read books over the summer. "Assign" them, a book list. Say, reading a book a week kind of thing.
Go to the library with them.
And have them write a short summary about the book.
Also get a composition tablet, and have them write a "journal" during the summer.