Hi, Sarah, I don't have kids and I don't sell cookies any more, but when I had my shop (and even now, as I am juggling many, many different projects at one time), I had labor standards (i.e., knowledge of how long each task took me) and therefore how much I could reasonably book/do in any given week. Now (and then), I keep a master annual calendar with all of my bookings, commitments, and other deadlines, and I try not to book more than my labor standards say I can do. I also keep a detailed daily production schedule that is mapped out at least a month in advance, so I always know how I am tracking to plan. I live by my lists. Without them, I would not be able to get as much done. Even so, I work an insane number of hours - to be truthful.
But, I think you could take on cookies if you have solid, uninterrupted blocks of time when your kids are at school and you are not working, and if you ruthlessly plan in some fashion similar to this. The key is first getting a grip on how long it takes you to do a typical cookie order or a certain number of cookies, and then blocking out that time so you're not getting stressed doing them.