I don't think you can. Other players have had this problem when they split up Sim couples. The best you can do is to send the child to the house where the couple is living. The child won't go home unless you send him home. Of course if you have four sims living in a house together there is no room for the child. You will need to move one couple to another house and then just send the child to that house.
If you want a single child to live with an adult, you have to raise their relationship with that adult. At some relationship status you get the option when socializing to "Ask to Adopt" from either the child or adult, or both. Can't really remember but I've done it before in cases where (oops) I let a child's parents die of old age before he was old enough to move around on his own. You can do this with a toddler or teen, too, I believe.
So it should work with the parents of the child as long as there is room in their household. So as the previous reply said, you would have to move the child's parents to a lot with room and then raise the relationship. I don't know how the child ended up alone, but this would fix it I think. As to how you move out the couple without breaking them up and then getting them back together in the new house I don't know. I've never been able to move a married couple only from a house with more than them living in it. I've ended up stuck with 2 childless couples in a house together and had to kill off a sim or break up a couple. I can move either a whole household or a single sim, but not just split a household down the middle.
The "ask to adopt" option doesn't work with the child's parents since they are already the parents. I tried it when I was helping someone else on this forum.
The he girl who games also did a blog on getting divorced that address the moving out issues.
