If it rains a lot where you are you should make sure to build the dog house high enough off the ground so it will not flood when it rains. One simple way to do this is to use a layer of concrete blocks as the foundation for your dog houses, as these will raise the house at least a foot off the ground.
If you are in an area where the temperature can go to extremes, either hot or cold, take this into account when building your dog house. You need to make sure if you're in a hot area that the house is well shaded to keep your pet from getting too hot. If you are in a cold area, make sure there are flaps over the door, to keep heat in, and a mat on the floor so your dog can stay warm while curling up.
Once you start building your dog house, there is a final detail to keep in mind. While you may want to paint the dog house to match your child's tree house, make sure you keep the paints to the outside of the house. Dogs are prone to chew on things and some of the chemicals used in paint can be damaging to their system if they were to chew on them too much.