HI. I'm using SL K with Readers 1 with my 5.5 and 3.5 yr old. Core and science, for math we use RightStart. At this point my 3.5 isn't really doing the core but he soaks things up just being in the room. We love SL here, but if you've read the reviews then you should know that "K" as a core title does NOT mean kindergarten. At the very earliest you should start it at around 5.5, but it's content is useful and challenging up through about 8yrs old, or (the end of) 2nd grade. If you're wanting to combine your children, and to be les stressful since you're babysitting, you might want to look into starting with the P3/4 program. I know it's hard to look at the program numbers and not think ' my kid is too advanced for this' but honestly all the cores are numbered very badly. It confuses a lot of people. As the children get older it would be easy to keep them in the same core without getting too far ahead for one and without being too easy for your other.
The only reason I personally am using the K program is due to finances, I've rescheduled it so that it will last us about 2yrs, we'll have time to save up for the next one by then.
Also, if you start at P3/4, you'll have plenty of room to add things in for your oldest if you feel he needs it and you'll easily be able to keep the kids together for as long as you choose to homeschool, customizing the math and L.A. Starting slow with good content will give you plenty of relaxed time to discover your kids' learning styles and your teaching style.