One of the fundamental problems in these situations is that most kids especially younger ones are not sufficiently independent/motivated and still need an adult when they are really learning new material. If you're looking at options inside the classroom that might be more task switching than is realistically possible.

I had an interesting conversation a year back along those lines with a teacher who had tried a totally separate set of activities for a few kids in the hall. She ran into the issue where they drifted back into the main group because the peer group issues i.e. feeling left out of the main classroom were too powerful.