Right, if you are applying for PR for your family, you must include your spouse and minor children and they must be medically examined. If they are deemed to be excessive demand, the whole family in inadmissible.
There are some very few cases of people getting around that by for example being so well off that they set up a trust fund in order to take care of their child's medical needs and signed some papers stating that their child would never be a burden on health care. However, very rare and very hard to do.
The family sponsorship is for people who are already PR or citizens and are sponsoring their spouse and/or children. In that case, the sponsorship can not be refused based on excessive demand. However, in order to do that, you would have to turn the clock back and apply for PR before you got married and before these children were born.