Are animal crackers that depict animals such as swine, lions, and camels treif?
Not because of its shape.
The laws of kashrus are concerned (in the case of cookies) with the ingredients and processing of the food. The shape is [literally] immaterial to this set of concerns and therefore does not affect the status of the cookie as edible according to Jewish law or not.
Also notable is the fact that the term "treif(a)" to refer to anything other than an animal whose life expectancy due to a wound is shorter than 12 months is somewhat of a misnomer.
Even food that is made to resemble non-kosher food can be kosher; see, for example, the hechshers on fake bacon bits, fake crab, Morningstar Farms fake sausage, etc. If these foods are still kosher, how much the more so for animal crackers which are clearly not actual animals?
(For reasons of marit ayin (giving the wrong impression), however, you should be careful in how you serve the convincing fakes.)
Technically, shape does not determine kashrus. An lion shaped animal cracker is not a lion, just some wheat in the shape of a lion (it's like saying that one who eats cookies in the shape of a person is commiting cannibalism).
However, according to Kabbalah, non-kosher animals come from the three impure klippos (shells). Therefore, The Lubavitcher Rebbe and here (a long interview on this subject is also here) made a campaign against having children seeing pictures of not-kosher animals as they are growing up. Therefore, perhaps one should avoid giving children such cookies, not because of kashrus.
Although the confection itself might be Kosher, I would not drink it with milk lest I make light of a mitzvah or diminish the importance of its underlying ethical precept.