But actually calling it a tar pit sounds....weird for their terms, i would go with the Sticky Water myself 
Yeah, that kinda bugged me, too. Maybe the filmmakers couldn't think of a good LBT-sounding term for “tar”, and since it wasn't quicksand, they didn't want to call it “sinking sand”. My suggestions would be “black goo”, “sinking goo”, or “black sinking goo”, but those might sound a little awkward when Little My suggestions would be “black goo”, “sinking goo”, or “black sinking goo” (or even just “goo”). However, most or all of those might be too long or too silly-sounding to insert in Littlefoot's line “You're stuck in a _____ pit!” While not a very LBT-sounding term, “tar pit” is short (you wouldn't want to have to use a long, hard-to-remember, and/or hard-to-say word to urgently warn someone of danger), and clearly tells the audience what the object is (for the benefit of those who hadn't figured it out already).
By the way, did you know that the “tar” in tar pits is actually liquid asphalt? (“Asphalt pit” doesn’t sound as cool, though.

And it sounds even less like a LBT term.)
As for the “sinking sand” in the second film, I'm guessing the tar-like appearance (perhaps most obvious when Ducky dips her foot into it) was intended to make it look sticky (In real life, quicksand isn't sticky, nor does it “suck you down” the way it's portrayed as doing in movies). On the other hand, tar pits often accumulate a layer of water on the surface, causing them to look like innocuous waterholes (Hyp demonstrated how that can lead to animals being preserved in them

), so you could argue that the “sinking sand” is in fact a tar pit; perhaps one that has opened in a location where runoff from natural erosion tends to settle? :unsure: (I’m unsure of the geological conditions that would be required for tar to acquire a layer of water and sediment on top, if such a thing is possible.)