A Canadian family of which one child has only a US passport, will they be bale to re-enter Canada?
I don't know about Canada.
I do know that in the USA you will not be able to enter with only a foreign passport as a citizen. The US validates the citizenship with the passport, and if you can't provide a US passport, you will only be able to enter as per the laws pertaining to the country of your passport. For example if you are a US citizen with only a French passport, and the usual protocol for entry from France requires a visa, then you will need to arrange a visa prior to entry with your French passport into the US as a US citizen.
I know of a family who's child was turned away from entry in the United States, because only that child had a non- US passport. They had to arrange for a visa for the child to be able to return to the USA, a matter of a couple of days. (A US family who's primary residence is USA)
Logic indicates that other countries would follow the same protocol, so I would suggest you contact the correct Canadian government offices to ascertain the law.