As noted in the linked duplicate question, your visa for Canada exempts you from the requirement to hold a transit visa. However, you cannot leave the international zone of the airport unless you hold a regular (type C) Schengen visit visa.
The legislation creating the transit visa exemption is Schengen Visa Code Article 3(5)(c):
The following categories of persons shall be exempt from the requirement to hold an airport transit visa provided for in paragraphs 1 and 2:
(c) third-country nationals holding a valid visa for a Member State which does not take part in the adoption of this Regulation, or for a Member State which does not yet apply the provisions of the Schengen acquis in full, or for Canada, Japan or the United States of America, or holders of a valid visa for one or more of the overseas countries and territories of the Kingdom of the Netherlands (Aruba, Curaçao, Sint Maarten, Bonaire, Sint Eustatius and Saba), when travelling to the issuing country or to any other third country, or when, having used the visa, returning from the issuing country;
The exemption is also reflected on the European Commission's page Who must apply for a Schengen visa.
The fact that an airport transit visa does not allow you to leave the international zone of an airport follows from its definition in the Schengen Visa Code (Article 2(5)):
‘airport transit visa’ means a visa valid for transit through the international transit areas of one or more airports of the Member States;
Nineteen hours is a long time to wait in an airport. It might be worthwhile to try to get a regular Schengen visa to allow you to visit the city, but with a whole family it is also perhaps less likely to be worthwhile than for a single traveler.
The useful site Sleeping in Airports has a helpful page on Frankfurt noting that it has good options for food, that there are airport lounges with single-visit passes available, and that you can use a shower for a small fee.
They also mention airport tours, but it is not clear whether you would need to be able to clear passport control to book such a tour. They do mention passport control and security in discussing the option of going to the city, so the tours may indeed be an option for you. If you find out while you're there, please come back and post an answer.
Finally, there is a transit hotel in case you want to sleep more comfortably than you otherwise would be able to.
A similar article at trip101.com notes some of the same things, but also mentions that there is a children's play area.