Medical is not required for Canadian born children.
Unless they aren't accompanying, you have to include them in the application, -- failure to do so might result in misrepresentation -- also in case of non-CEC/ without valid employment applicants, you have to show funds even to support your Canadian born children (assuming they are minors).
There is a glitch in express entry application system which requests RPR fee payment for Canadian born accompanying children --even after indicating that their place of birth is Canada and citizenship is Canadian.
I faced the same issue and I proceeded by paying the fee and explaining the same in a cover letter. I received PPR without any issues, mailed passports and waiting for a refund.