A very important aspect for oral motor therapy with babies/young children with Down syndrome is discouraging tongue thrust and encouraging lip closure. A very simple way to get this "oral motor therapy" in is in the way of spoon feeding. There's a certain technique which can be done when spoon feeding to facilitate this.
I've posted about this on the blog years ago, back when we were doing this with O (when he was a baby), and it's also on our site, but I thought I'd post it up here again.
This can be used on children who are very young - from the very first time they start eating solids!
Hold the spoon sideways, so that the side of the spoon is touching each side of the mouth, let the child get the food off it that way.
Immediately turn the spoon the other way (still sideways, but facing a different direction) and feed the child.
Then turn the spoon again the opposite direction and let them get the last bit of the food off.
It is best for this to be done 3 times in a row, like the above picture shows, because then it helps keep that tongue in. If you just do the sideways spoon feeding once, then the child can put his tongue out to help swallow the food or lick the food off of his lips. After you do it 3 times the child will hardly stick his tongue out at all after that last bite, since he has already been swallowing the food.
Try it on yourself - Feed yourself with the spoon like you normally would - your tongue protrudes forward this way. Feed yourself with the spoon sideways, your tongue is pushed back. It works so well.