I've done this heaps of times, without a problem, on international multi-leg flights, with a pre-schooler in a seat, and a toddler in my lap, all by myself, and as long as you're prepared it won't be a problem.
As has been said, have spare clothes and daipers (for your toddler, and a spare t-shirt/top for you); and make sure you have a favourite drink/sweet for them for take-off and landing so they keep swallowing (it stops their ears from hurting). It was one of the few times I'd happily hand lollipops to my kids, and let my toddler have juice from a bottle. If they're a movie/DVD kind of kid then a portable player should keep you both happy, but bring whatever your child's favourite plaything is along with you. Drawing things probably aren't gonna work as you may find you don't have enough space to lower the tray table with your toddler in your lap.
Just keep in mind to pack your carryon carefully - you have to handle it and your child. I had a reasonably large sized kapoochi backpack that had lots of pockets and compartments, that if I packed carefully would hold the neccessities for the three of us. Because it was a backpack, I had both arms free to look after my kids. Best hint I can give is to pack the extra clothes in ziplock bags, then suck the air out of them so they take up less room - if you need them, you have a bag ready to put the 'dirty' stuff in.
If your child is going to need motion sickness meds, beware, as quite a few of them (especially phenergen) can have the opposite effect on some children. (A friend of mine had to do an 11 hour flight with a hypo 3yo son after giving it to him.)
Finally - relax and HAVE FUN - you're going to WDW afterall.
