Writing the dialogue of a toddler

I'd say go with the dialect. You could also translate a phrase or two, as the dad mentally translates. So, for instance,

"I div yoooooou." I love you. Lincoln bent to kiss his son's forehead, and the boy repeated, "I div yooooou da da."

Edit: I'll add, think of how we gain some insight into Groot's speech simply by hearing Rocket's responses. So you don't have to mentally translate everything, just have the dad respond in a way that lets us know, after-the-fact, what the kid is saying.

I favor using the actual toddler dialect because, a) it is realistic, and b) it's almost always cute. Maybe I'm a sucker for it.

 

It really depends on your preference and style. In the Harry Potter series everyone has an accent but the only accent that we can literally read is Hagrid's. The neglect of some consents and the placement of apostrophes lets us read his accent for everything it's worth, we can read it aloud and speak with that thick, rugged accent.

Other times I prefer to write the accent in the narrative. One character in my WIP is a kid with missing teeth so I write he has a missing-tooth accent. Another has a southern accent, so I wrote, "he spoke like a cajun hillbilly,". After that I wrote in full words just like this and let the description carry into the dialogue. It's worked so far, the people who've read the story got it.

My advice is, take a scene with the character and write it two ways. One with the dialogue written with accent words, like Hagrid, and the other with the accent described but the sentences/words written in whole words.

 

It may just be a personal pet peeve, but I HATE when kids' dialogue in books is written out in baby talk. I couldn't precisely say why. It just really rubs me the wrong way. And I have 5 kids. I've been listening to that kind of speech for 13 years.

 

I'm with Mythopoet. I'm fine with accents and speech impediments being written out to a certain extent, as with the Hagrid example; contractions and some mispronunciatons aren't too jarring to read. Now when authors are butchering every word spoken differently, to the point of "I div yooouuu" or how Stephen King writes some minor "country" characters, just to drive in the point that the character speaks differently than the author, I find myself kinda skimming over those parts because they're too irritating to read. It's actually kind of offensive. Yes it makes the accent pretty clear, but I'd be fine just being told the character speaks with a rural drawl and shown a few apostrophes in the dialogue. Same goes for toddlers. Say a few da-das, maybe "I wuv you" might be a little less jarring than "I div yooouuu", but if the dialogue isn't mostly coherent English, just have the father reference that the rest was in jibberish only he can translate.

 

I think it depends on how much direct speech is being used for the scene, what role the dialogue plays, the type of conversation being had, and what the author wants to show in the interplay between characters.

Ignoring the toddler's direct speech may distance the toddler, who then becomes more of a prop or plot device, something like a familiar pet dog referenced in the scene only to show something about the MC when the MC reacts to its presence. On the other hand, an extended conversation written in real time with direct speech between the two could become annoying if large portions of the toddler's speech are extremely incomprehensible.

There's no reason a scene must fall into either of those two extremes.

Edit: I'm not saying that neither of those approaches is ever useful. It just depends.