I gotcha here.
Set up a system exchanging reading for screen time. Take them to the library weekly so they always have fresh books. Talk to the children's librarian when you go for new recommendations. I'll start you off here - get your fifth grader the first Warrior's book Into the Wild and your first grader Mercy Watson and Dog Man. I have my fourth grader read a chapter book in exchange for screen time, but my first grader can read anything she likes. I limit screens pretty heavily, but they need to read thirty minutes on weekdays in exchange for thirty minutes of screens. But if your kids are used to more screen time than this, something like the following will work:
Screen time can start after you complete the following:
Read for thirty minutes. Active or creative play for an hour. Outdoor time for an hour. Chores are completed.
In my household, active/creative play includes building forts, building with LEGOs, drawing, "cooking", board games. It's mostly time spent entertaining themselves away from screens. Outdoor time can even include just reading their books out-of-doors, or going to the playground, or digging holes. Feel free to adjust the times to whatever works best for your family, but I'd stick firm to thirty minutes of reading and at least an hour of entertaining themselves. You'll find that after a few weeks, the games they invent for themselves often turn into much longer than an hour.
For math, you can get your kids the Summer Bridge workbooks as someone else recommended, or you could just play card games with them all summer and make it a ritual. There is a lot of math to be learned. For first grade, War is a great game because it deals with number/value recognition. Yuker or Cribbage or Blackjack or Poker are all great for fifth grade. I'm a huge board game fan and I'll give you a million recommendations on more mathy games if you'd like, but you have to be up for playing with them. If you don't have that time, something like this is just fine.