I am also in the situation of having grammar stage kids and a logic stage kid. What We'll be doing next year is to use Elemental Science Logic Stage Biology along with their Lapbooking Through Biology for my youngers. The logic stage (and grammar stage) programs are basically science done the WTM way all planned out and organized for you. There are weekly plans, sketches, "experiments", memory work, and some optional quizzes. Lapbooking Through Biology has the lapbook stuff to print out, vocab, planned reading suggestions from different books (though she also has the readings scheduled out from just Usborne Science Encyclopedia in a file too). This way we're all on the same subjects but working into their own levels. It's also not super expensive ;)