Group portraits are challenging. Everything must tie together into an integrated whole, with each person's rendering a success. In this case, the tree and landscape are used to tie the space together, and human contact, the baby in a loving sister's arms, makes an affectionate connection. The young man's eyes gaze at the others, carrying the viewer's attention from one face to another. The youngest boy is a bit impish, and looks ready to run off and do something really fun.
There are some who claim that children's portraits should be dignified, still and quiet and that a smile does not belong in a portrait. I'm not one of those. I believe the reason few artists paint smiles is because it's very hard to do it well. Or that no one has been looking at the Dutch Masters nearly enough. Van Dyck, Judith Leyster and Rembrandt often painted laughter! Children love to laugh, and so do I.