Is it safe to give a 3D printed toy (printed using food-safe filaments) to a toddler? Are there any recommendations/studies on what is considered safe?
The answer to this is highly dependent on the geometry, slicing settings, etc. Clearly it's possible for the 3D model itself to have small parts that would be choking hazards, or to have parts of the geometry such that, due to being thin or providing leverage, allow small parts to be broken off and become choking hazards, sharp, pointy, or otherwise dangerous. It's also possible that a print with insuficient layer bonding or poor infill could be split at layer lines or chewed apart, exposing parts that might be likewise dangerous.
If the part is designed with these safety considerations in mind, sliced properly, and printed without error on a reasonably well-tuned printer, it should be safe against mechanical hazards.
The IC3D Toys for Tots campaign might give you some ideas about the types of 3D printed toys a reputable organization considered safe for children. I'm not sure right off what age groups they targeted and whether all the toys in their campaign would be appropriate for toddlers - probably not.
The model election is a non-trivial issue when dealing with to a toddler but also, the extruder and hotend should be 100% free of debris from any previous, non food-safe filaments, although I don´t know how plausible that may be from a procedure point of view and I don´t know if there are any regulations with thresholds on this, but it´s not like we will be making a chemical analysis of a print to verify neither, so moving on to next point
Depending on what you want to do (and your printer flexibility), you might as well have 2 sets of extruder/hotend/bed if your really want to go this route on a production scale: one for food-safe prints and the other for the rest of the filaments like ABS