This is one of those questions I don't like. Usually when people start asking me whether they should have children or not I change the subject. Then I say, "Go and talk to Steve." He is usually much more open about what he thinks about these topics.
Certainly there are a lot of children in the world who would like to have parents. So, if you are considering having children in this way (adopting a child), it would be nice for a child who doesn't have parents to finally find some parents.
One retreatant told me something I had said to them when they asked me this question recently. I'll paraphrase them, maybe that will be interesting. This particular person is very interested in the Dhamma and developing themselves, and I said to them, "Why don't you grow up in the Dhamma first before you bring another child into existence". I was talking to someone who is very dedicated to the Dhamma so, for that particular person that was appropriate.
Is bringing another being into existence just more Dukkha? Certainly we have to consider the amount of time and energy that we're going to have to give to this being, before we actually take on the responsibility of having a child. Many people actually don't consider the amount of responsibility and the amount of time they have to devote to this being, if they are going to actually do a good job at parenting. If you bring a child into existence, you have to take responsibility for that child and give them the amount of time and love they need so that they can grow up in a good way. Sometimes people don't take into account this amount of time and are not really willing to devote the amount of time necessary to give the child a good upbringing.
So, if you're going to have a child I think you've got to actually consider the amount of time and what you want to give up in order to fulfill that responsibility. I think this is one of the problems that people are having these days, they are bringing children into existence and then not willing to give them the amount of time that is necessary to give them what they really need. They are often too interested in their careers and bringing in the money; giving them things that don't really nurture the child's development, and are often not willing to give them the time and the love necessary for bringing up the child in a good way.
It is important to also consider not just the happiness of having someone who is "mine", but also consider the other side and ask ourselves whether we are really willing to give that amount of time and effort for another being. And considering the amount of humans in the world maybe it is selfish - I am not quite sure. But most people usually don't think of that. As far as the opportunity for someone to choose wise parents goes, I think it is our Kamma that decides that, what parents we go to.