You might want to consider homeschooling. Otherwise, your best bet is to provide enrichment, but that doesn't have to mean "homework". Keep in mind that any acceleration (teaching them standard curriculum at a faster pace than their peers), will compound your problem later. While distance learning undoubtedly makes differentiation less likely, advanced learners were often left unchallenged even in school pre-COVID era.
If you do decide to homeschool or accelerate (or even to catch any gaps caused by DL), the series What Your _______Grader Needs to Know by E. D. Hirsch, Jr. , provides an excellent general reference.
The Hoagies website has an area that offers links to fun enrichment for kids in all subjects.
https://www.hoagiesgifted.org/links.htm
Brainpop has animated lessons on a multitude of topics.
https://www.brainpop.com/
The National Library of Virtual Manipulatives lets kids play with and explore math concepts online.
http://nlvm.usu.edu/en/nav/vlibrary.html
Sir Cumference books by Cindy Neuschwander provide an introduction to geometry topics.
For history, you might try the Timewarp Trio series, the Magic Treehouse series, and Ben and Me. I've heard good things about a series called Horrible Histories, but have no experience with it.
Play games. They almost always have a math component (keeping score, making moves, etc.) and encourage logical thinking. Some, like Sleeping Queens and Monopoly, incorporate math more directly.
Logic Puzzles are great. DD loved Rush Hour. Magic Squares and Sudoku are also good.
Mathisfun has games, puzzles, and explanations of math topics
https://www.mathsisfun.com/games/index.html
Cooking is excellent for fractions. For added challenge, you could double or halve recipes.
Crafts can be great for math. Many projects involve measuring, calculating perimeter and/or area. Origami helps with geometric thinking.
Give the kids tape measures. Timers can also be fun.
Secret codes might be interesting for them. Seymour Sleuth mysteries by Doug Cushman are picture books featuring codes and puzzle clues. The Third Grade Detectives series by George E. Stanley explores secret codes in the context of chapter book mysteries.
Encyclopedia Brown and Einstein Anderson are mysteries for the reader to solve.
Madlibs is great for an introduction to parts of speech.
Schoolhouse Rock
Freerice is a program that donates rice as you play their quizzes. If your kids are charitably inclined, it might motivate them to work on vocabulary words. There are other subjects as well.
https://freerice.com/about
TV can be a great asset. Magic School Bus is excellent for Science. Cyberchase is great for math (although I think the focus has shifted in recent seasons, I think old episodes can still be found on the PBS kids website.)
https://pbskids.org/cyberchase/videos/?selectedID=331ac4a3-bfe1-4ef1-b6e4-db8204f97796&episode-id=2883225c-0ec9-4490-9f88-7890cfb1689a
PBS shows are generally good.
My kids loved Zoombinis and The Incredible Machine computer games.
If your kids are interested in computers, Scratch is an excellent introduction to programming. It's a programming language designed by MIT to teach programming to kids intuitively.
https://scratch.mit.edu/parents
If your kids are interested in foreign language:
https://www.youtube.com/channel/UCyY3Wd5x85o8AKXjYSoxFAQ
https://www.Duolingo.com