There are a lot of things to take into consideration, the most important one that most people forget about is this:
What are you going to do if your child winds up allergic to the dog?
This question isn't JUST about the toddler. Yes, the toddler needs a protective dog, that isn't overprotective, the toddler needs a dog that can keep up but won't "over-play" and a dog that is strong enought o let the toddler play with the dog without harmign the dog or the child.
Here's another question for you, this will be the child's first pet, how long is it going to live? Are you prepare to answer questions about illness and death to a child? If not you might want to get a mixed breed dog, their lifespans tend to be longer than purebreds.
Another trick is to make sure you have the toddler WITH you when you pick out the dog. Let the dog pick the toddler. If the dog is too interested in the adult they may become jealous or try to dominate the child. Clear pack ranking is a MUST. This first encounter also allows you to see if your child is afraid of a particular breed of dog AND if there seems to be immediate alelrgies present. You still need to keep in mind that the child may develop allergies and have a plan in place but this is a much better solution than bringing home a dog and having the little one begin wheezing.
In my life mixed breeds worked out best. They lived longer, they had less health problems in old age and tended to be more even tempered (I have NO idea why that is because some breeds are bred for temperment). Working classes did well, sheparding dogs, especially Australian and German were good with children, Collies tended to be a little too high energy and a little too smart, they get bored with a simple task and believe it or not, watching a toddler is easier than watching a herd for a dog.
If you've an active family consider a more active breed, if this is supposed to be a dog that does nothing but play with the baby get one that's more sedentary.