I understand where you are coming from! I have a 2 year old, K and 3rd grader. There are days when I certainly feel very overwhelmed and that nothing gets accomplished.
We are slowly starting to work out the kinks in our daily routine. What is working is that I combine some lessons for the K & 3rd grader. Story of the world, art lessons, US History...just to name a few. The more I can do with them together really does help. Also, I have "focus" days. For example, on MWF's I will do more hands on with my 3rd grader. On T,Th's I will do more hands on with my K student. Every day, they have something to accomplish/ things they can work on independently while the other is getting my full attention.
We have been doing this for 4 years now- and although I have only been homeschooling 2 children at the same time for one year, I can assure you it takes a little time to work out the kinks. It will get easier! Hang in there!