Yes, the space family is always a family of one child. My current 2nd generation tama is an only child, and she's currently the Mikakuzitchi. She'll become a Sunnytchi and go on to marry another tama, but there's always a chance that the next generation, unless another Sunnytchi married my own Sunnytchi, will have more than one child. If your tama, whether a Sunnytchi or not, marries a Sunnytchi from the Dating show, there will only be one child.