Found this info that might be helpful...
A high-fiber commercial food will work, or you can mix unflavored Metamucil into canned food. Check the label for the type that supplies 2.4 grams of soluble fiber per dose. For dogs, use 1 teaspoon per meal for each 10 to 25 pounds of body weight.
So you may want to mix under 1/2 teaspoon.
Another thing you can use is Grape Nuts cereal- dogs seem to love that as well. Just give a few teaspoons of it a couple times through the day and that should help your pet "go".
I'm sure they'd love it as a treat, as well.
Hope that this help, Shana...worried about your little one!
http://www.sidyboysfoolin.com/Constipation.html