A single parent
My honest opinion is that the loneliest people are the children who are brought up by a single parent. A least, an adult person can find another partner, another lover, another husband, but a child can not find another father or mother. Of course, in the happiest cases a child finds a missing parent in a step-mother or a step-father. Everything is possible, but with some conditions.
The first condition is that a child in a partial family needs more attention. And this burden is laid on the single shoulders. Nowadays, a woman often makes a decision to raise a child alone. So, she is to be ready to the responsibilities. In other cases there is not another way for a single parent. Only to be more thoughtful towards a child.
Another item to speak about is a personality of a missing parent and an idea of a child about him/her. "He is very far" is the most popular and neutral theory. It seems to me that the worse idea is to make a child hate or dislike an absent parent. It is better to leave everything for the future when an adult person will be able to make his/her own opinion and decide what to do – to communicate with a father/mother or not. But a real story of the parents' relationship and the reasons for their parting is to be explained to a child or a teenager in the very clear and simple words, perhaps, even without some details.
A child without one of the parents could feel uncomfortable, because other children have both a father and a mother. Try to suggest a child an idea that there is nothing awful in his life, that you are happy to care of him, support and expect for the same. Because two of you is a family, but the families are to be thoughtful, caring and happy.