@Selkie: sorry if I came across too strongly, but I did feel the heavy hand of the PC police on this matter when I attended classes at a university in California in the late 1980s. Since I believe in a more descriptive approach to grammar, I did not take kindly to authority figures trying to ram their preferred grammar structures down my throat. If people want to use "inclusive language," that is their choice. And it is possible that at some point it will become the dominant and preferred form. However, I do not believe that most people object to the indefinite masculine form until they have been "instructed" or "taught" to take offense at it. As it is, English is far less sex-aware in its grammar that most languages. I always found it amusing to see rigid proponents of inclusive language in English speaking Spanish (common among the multi-culti crowd in California), which is far more demanding in the assignment of sex to every noun and adjective, and typically uses explicitly masculine forms of words as the "norm" (e.g., "padres" for "parents").
When I first read your posting, I thought that perhaps you were the social worker evaluating the application, hence my urging a generous approach. Since you are actually helping the prospective parents to improve their application, the approach suggested by HM and outlined in your own response is probably appropriate.
Just as an aside, HM's suggestion to use inclusive language because some social workers are liable to take offense at non-inclusive language is, to my mind, further evidence of the heavy hand of the PC police. The applicants are tailoring their language usage to avoid offending a person who has power over their lives. Unfortunately, given the assymetrical power relationship, they probably have no choice. I would probably do the same.
One other option is to give the unknown child a generic name that could be either male or female and use that name throughout the application. My wife and I did something similar when she was pregnant. We did not want to know the sex of the child in advance and so needed a way to discuss the child with medical personnel, most of whom were already aware of the sex because they had reviewed sonograms. So we called the baby "Dinkelbert," specifically chosen because of its comical sound and extreme unlikelihood of actually being used.
Dinkelbert did routinely received the masculine pronoun, although if we were discussing the specific possibility of a girl, the name might morph into "Dinkelberta" and the pronoun to "she." Curiously even before we mentioned our technique for discussing the unknown child, the almost exclusively female hospital staff (whom my wife saw twice a week for most of her pregnancy) informed us that they would always use "he" when referring to the child and that we should not assume anything by the use of the masculine pronoun. As it turned out, the baby was a girl (as the nurses, technicians, and doctors knew from almost day one).