D.B.
The best thing you can do is recognize that learning entails many things. Standardized tests are not the only measure of intelligence or mastery of certain subjects. The mark of an intelligent person is one who is curious and can engage in critical & creative thinking, not just one who knows facts. Allow your children some down time to explore - whether it's outdoors or with toys or books or puzzles. Be sure to engage all their senses and multiple skill sets. Take them to museums, take them bowling (exercise and math), take nature hikes (look, listen, smell, touch), do crafts based on the seasons. Take time to smell the roses, you know. DO NOT overschedule - those kids cannot do anything on their own without a structured system and a set of directions, and they aren't really comfortable with themselves. When I see people walking their dogs, pushing a stroller or hiking while talking on a cell phone, I go crazy! Stop, enjoy, slow down, experience the world around you!
Give your kids interactive toys that encourage creativity - things like legos or brio trains or marble-building sets. Let them mix & match toys - my son built Lego and K'nex buildings over his wooden train tracks, added hot wheels, and so on. He's now studying structural/civil engineering. He had ONE activity plus religious school per week - sometimes it was basketball, sometimes soccer, sometimes an after-school club. The rest of the time, he PLAYED with friends - he learned to set up neighborhood games, he built stuff in the woods, he learned to socialize, etc. We went to museums and to nature areas. He had down time when we didn't organize him and he was responsible for himself. His first intense activity was in high school when he joined the track team, and the first year the coach really encouraged the kids to try different events until they found the one that suited them. It was the greatest experience.
We also played board & card games - Monopoly, Sorry, Scrabble, Uno, Boggle, Clue, etc. They encourage reading, math, problem-solving, spelling/vocabulary and social interaction -- without the kids knowing it.
I would still have your kids have quiet time/reading time before bed - it helps later on as their stress level increases.
By contrast, we have a neighbor who spent all his time on the computer and taking advanced placement courses. He was miserable, he never had time for friends because he was pressured to score well on tests and in classes, he has no social skills, etc. He scores as "smart" but he has no life. He had no activities and he didn't wind up getting into that great a college because he wasn't the well-rounded kid the schools wanted. This was not the fault of the public schools, by the way! So if I were you, I would lighten up a little, expand your definition of "learning" to include passive learning, and be sure your kids are independent, not resenting you for pressuring them into specific activities.