Be the example to your kids. Always smile at the, hug them, and leave work issues at the door. Tell your kids how much you love them and they love each other. They will buy it. Kids are naturally happy.
Swiped from yated.com:
A few of the men walking with the Rov ventured to give voice to their thoughts. Perhaps it is because children lack a maturity which brings with it a more solemn outlook on life, one suggested. Perhaps it is because they have few responsibilities, proposed another.
The Rov shook his head, indicating his disagreement with these suggestions. Realizing that the Rov wanted to teach them something, the talmidim stopped and waited to see what he would say. Why, indeed, are children normally in a happy and carefree state of mind?
As we grow older, the Brisker Rov explained, we complicate matters. We erect mechitzos, barriers, and we bury our natural state under layers of self-imposed encumbrances (such as jealousy, desire, greed and doubt). Children, who have not yet complicated their lives with external baggage, are in their natural, purer state.