It depends in what context you are talking about the child. Is it a word the parent(s) use between themselves? Do they say it to the child? Do others use the word about the child? When I found out I was not planned, I said to my mother, "I was an accident," and she said that no, once they knew they were having a child I was just as welcome as the others. If someone called a child unplanned or accidental or unintended, that would automatically have an implication of being unwanted, at least to the child. To a child I would say "happy surprise."