It's all about costs. The ruling case law in this case is the Hilewitz decision (http://canlii.ca/t/1lsvm) and is on point with respect to social service requirements. More recent decisions (e.g., Companioni - http://canlii.ca/t/2795j) have extended this to medical costs as well.
If the anticipated will exceed the "excessive costs" threshold (adjusted annually by CIC and last I looked more than $6k per year) then CIC will refuse your application on the basis of "excessive demand".
This is a quagmire for CIC to be honest, because Hilewtiz established that CIC must conduct an individualized assessment of the prospective PR. This is an area in which officers are found to be deficient - I've read dozens of these cases and judges are deeply troubled by the policies of CIC in this area and have constrained them - including rules around fairness, clearly establishing the basis for a negative decision (e.g., Sapru) and properly evaluating applicants.
But this is a very specialized area. The OP has entered the first part of this process: evaluation. CIC will want to establish how many social services will be required to determine if the child will create excessive demand (in the next five or ten years). But this process is rather subjective and an applicant can obtain alternative opinions that disagree with the medical officer.
If the medical officer reaches an opinion that the child is inadmissible due to excessive demand, she or he will recommend refusal to the visa officer. The visa officer will then issue a "fairness letter" advising the applicant of the issues and giving the applicant some period of time to address them. An applicant can disagree with the medical diagnosis and the estimate of costs.
An applicant may also provide a "credible plan" for paying those expenses that does not rely upon social services. For example, if social services in the settlement province are financially based, an applicant can use those limits and point out that some (or all) costs will be born by the applicant under the rules of the province. An applicant can arrange for other options that may be available - but CIC will not tell you about these options. It is the responsibility of the applicant to do the research and figure these things out. For example, an applicant might be able to post a bond with the province, or use existing insurance to cover those costs, or find an NGO that is willing to bear those expenses. While CIC will also request an affidavit from the applicant that use of those services will not be required they won't consider it as part of their analysis.
There are a very small handful of attorneys capable of helping you in this area. They do an excellent job, but their services will cost thousands of dollars and there is never a guarantee - but working with an attorney will often put you in a position so that in the case of a negative decision you have a strong platform from which to challenge the decision - I'd say about 75% of skilled worker applicants who challenge their refusals for excessive demand in Federal Court are successful. That's a stunning number, given that CIC wins something like 80% of all Federal Court applications filed against them.
I've personally been through this, though in the medical excessive demand, not the social service excessive demand, but the process is the same now.