Would the consumer turn into an adult sized baby? Would they retain their powers of speech and motor skills but physically become a baby? Or would they just be transformed into a baby?
TLDR: Yeah, you'd fully transform into a baby, but you'd retain your mind and presumably your speech capacity.
While there isn't an specific instance of someone drinking a Polyjuice Potion brewed with a baby toenail (or hair, or whatever), there are multiple instances of the Polyjuice Potion being used, and we know the effects.
The Polyjuice Potion gives to his drinker the complete physical appareance of the human who gave the final ingredient. That includes stature, weight, and even genitalia. Thus, if someone drinks a baby-Polyjuice Potion, he will turn into a baby, including size. This means that you won't be capable of moving by yourself (since a baby hasn't developed his muscles yet, and can't walk).
For example, when the Golden Trio infilitrated the Ministry of Magic, they used the Polyjuice Potion to turn into diverse workers of the Ministry. In particular, Harry turned into Albert Runcorn, and Hermione had to give him bigger robes, because he was very tall:
Here, he's pretty tall, you'll need bigger robes...
Harry Potter and the Deathly Hallows. Chapter twelve: Magic is Might
However, drinking the Polyjuice Potion doesn't alter your mind. You retain your knowledge and don't adquire the knowledge of the other person. Thus, although you will be physically a baby, you will retain your knowledge. Meaning you might be capable of speech, depending on if the baby has physically developed the phonetic system.
For example, Harry and Ron ingested the Polyjuice potion in order to turn into Crabbe and Goyle, but they didn't know where the Slytherin common room was. And evidently, they kept their memories and knowledge.
All they needed now was a Slytherin whom they could follow to the Slytherin common room.
Harry Potter and the Chamber of Secrets. Chapter twelve: The Polyjuice Potion
There are another instances of Polyjuice Potion being used that could be added to offer further proof, but in my opinion this will suffice.