I don't use online curriculum, but I think my suggestions will work regardless.
I have books, toys, and daily activities planned for even the toddlers during the school day. It's great preparation for when they finally transition into schooling "full-time." When the older kids start school, the smaller children have a routine as well. I've already transitioned young ones into regular school from "pre-school" and it was very smooth...thanks to our pre-k routine.
Formal school time shouldn't last longer than half an hour each day. I spend about 5-10 minutes per subject, then let them free play with their educational toys, art supplies, and videos.
Daily, they learn their alphabet (any flashcards or workbooks of your choice from stores like Target or even a teacher's supply store are fine), practice handwriting (I used handwriting templates purchased at Lakeshore Learning and the Pre-K and K writing curriculum from www.hwtears.com ), and do simple math ( any workbook of your choice ), and reading/phonics activities.
I use Hooked on Phonics see: www.hookedonphonics.com and the book "Teach Your Child to Read in 100 easy Lessons for Pre-K reading. See: http://www.amazon.com/Teach-Your-Child-Read-Lessons/dp/06... .
I also provide lots of art and craft supplies and let them create freely most of time. Sometimes I have an actual pre-planned art project, but usually I have all the kids participate in that case. Lakeshore Learning store ( www.lakeshorelearning.com ) and Discount School Supply ( www.discountschoolsupply.com ) are fantastic go-to resources for pre-K learning toys, games, and art supplies in bulk.
Do not mix the school toys, books and games with the regular toys. They are for school hours only. You'll be amazed at how quickly they get used to the routine and will happily look forward to doing their own "school time."
It is not realistic to expect them not to distract you or bother the older kids. Have your arsenal of distractions if you need a few minutes quiet time with an older child. I had lots of luck with educational videos. The Rock and Learn Series is very good (see: http://rocknlearn.com/ ), Disney's Bill Nye the Science Guy (see: http://www.amazon.com/s?ie=UTF8&page=1&rh=n%3A###..., DK Eyewitness Science Series (see: http://us.dk.com/nf/Browse/BrowseStdPage/0,,231760,00.html ), and so on.
My toddlers particularly liked the Scholastic Book Treasury series. You can have the read-along option and the kids do learn while enjoying popular children's classics read to them. The animation is lovely and the stories are all books most schools introduce to early readers in grades Pre-K through 3rd grade. (See: www.newkideo.com )
Last but not least, the two following resources are handy for simple and easy lesson ideas and guidance.
http://books.coreknowledge.org/home.php?cat=298
http://www.welltrainedmind.com/
I have found having a special set of materials and a special table/desk and play area set up for the younger children works very well. I don't force them to stay. They will wander around in other rooms and so forth, but if you secure the house in case you get distracted and start a routine for them, they will in time follow through. Be sure to have plenty of breaks between lessons for outside time or to stretch, and offer everyone at least one snack to keep their focus strong as well.
And yes, there are bad days and crazy days even with this. As Riley mentioned you can always change things around and pick up lessons later in the day or if you need to tackle tougher subjects that require your full concentration, do so when the toddlers are napping. This is the beauty of homeschooling for those not using a rigid program... you can do it anytime and anywhere. There is lots of room for flexibility and that means lots of room for success.
One other interesting note...apparently my younger children listened to the older children's lessons..even though it appeared they were playing and making a mess at their desk. My now 2nd grader learned the mulitplication tables while he was in Kindergarten and by 1st grade was proficient because he heard and memorized by osmosis as the older kids practiced their math facts. LOL. So there are truly benefits in keeping the little noise machines around. In time you will find it isn't that bad at all and everyone finds their groove. It will all work out...so don't worry.