With kids as young as yours are, my advice would be to simply work into 'school' slowly. You have three weeks until you want to start, right? This week, start with the basics. What are you doing to teach reading and math? What supplies do you need to gather? Even if this is all you do for the first few weeks of school, you will be fine. In public school in first grade, most of the first month will be spent on transitioning kids from kindergarten to first grade, helping them get used to sitting for longer periods, reviewing basics, setting up classroom rules and standards, etc. You don't have to have everything perfect.
Then, next week plan another subject, or two if you have time. Handwriting/printing/copywork and Literature would be my next big ones, but you can decide.
The third week, figure out science and nature study.
The 4th week, start school with what you have. Figure out history.
5th week, add in history to your school days, and plan the 'extras' (composers, artists, poetry, etc.)
6th week, add in the new areas, plan your scripture memory system,
Don't panic, just ease your way into it.
I actually did this, except I started in March (when we decided we were going to homeschool and I settled on Charlotte Mason. I figured, why wait?) and then just introduced one new subject per month. I think it was about November of before we actually had everything going.
Since then we have been working year round, so I only do specific planning when we are close to finishing up a book or topic and need to move to something new. Much less stress!