Under the circumstances you describe, I'd encourage you to keep having them participate in one rec sport. My son is EXACTLY as you describe you kids. He's 8 and I'm glad I kept him in rec soccer. It makes it easier for him to keep being mediocre, learn the rules of the game, and join in on games at aftercare or recess when he's in the mood to do so (which is only about 1/4 of the time).
Oh sorry, I missed that your kids complain and don't want to go to practice. My kid is neutral about it - not looking forward to it, but not complaining either. Just neutral. So I guess the situation is somewhat different.