Without reading other answers: yes, do sit down and write out a tentative daily routine which takes into account breakfast, your shower, tv time (if any) morning snack, lunches, activity times, etc. Be sure that the kids get plenty of time outside/out of the house, too. If you have a specific daily activity that you want to focus on, be sure it's around the dame time every day. For example, while your daughter is napping, that might be the time to do a little skill-building with your son, then ask him to take a quiet time break for 30 minutes before sister wakes up. PLAN BREAKS FOR YOURSELF. I can't stress this enough-- be sure you have a couple chunks of time a day, because unlike working outside the home, there is no lunchbreak, no uninterrupted set amount of time--you will have to ensure it happens. (I was a nanny for years, so I say this as both a mom and a professional person working in someone's home-- that was the biggest adjustment I had to make when I went from teaching preschool to being a nanny-- figuring out how to create break times.)
If you can do the same things around each time every day (and at least, try to maintain the *order* of those events) things will go more smoothly. The kids will begin to predict the routine, which is good.
You also want to make sure you have a chance to get housework done throughout the day, too. Plan regular times for the kids to pick up their toys/activities (perhaps before lunch and a half-hour before you start dinner, do a bigger clean-up). When I was a nanny, I could set the kids down at the table with playdough toys/tools and a book on tape/disc while I washed the days dishes after lunch, or give them construction paper/sicssonrs/glue sticks, or stickers and paper and markers and let them work on their own.
For outing days, depending on when you get up, do your sandwiches/pack most of your picnic the night before, or get up earlier to do it. Go to the zoo or Children's Museum as early as they open. You'll get the kids when they are fresh, (the animals at the zoo are often more active first thing in the day) and you'll avoid the crowds that start to come around 10:30 or so. Earlier, from my experience, is always better. Esp with memberships-- you can ask each of the kids what it is they most want to see that day and just focus on those two animals, knowing that you can go back anytime. You might even choose to leave and go someplace else for lunch, like a shady park.
All this to day, I find that planning allows me, more often than not, the reassurance that I will find time to get those things I need done, done. Also be aware that 4-6 is an afternoon 'witching hour' for kids, so get your grocery shopping done in the morning or early afternoon. I'd rather bump doing dishes to a later time of day than take a tired kid grocery shopping in the later afternoon-- heck, I'd rather poke myself in the eye than take a kid grocery shopping then!
Have fun with your kiddos!