"(and find out how you managed it)"
We made meeting times for meals, excursions,other group activities, etc. You will need cell phones(or email devices) for communication, change of plans, which is very important. "The line at the eiffel tower is long and we will be an hour late for dinner". We woke up early and are out for a walk, will return at 9AM". "The kids had to eat now, we won't be back for breakfast."
Nothing worse than keeping the group waiting.

France is another good country for renting houses (Gites). Last year we rented a house in the Dordogne, an area full of family activities-caves, cave art, castles, Cro-Magnon man, gardens, markets,medieval towns,most beautiful picturesque villages, river activities(canoes, kayaks, boat rides), pilgrimage sites, cycling, abbeys,medieval rock shelters, and more foie gras than you can stand.Medieval Sarlat, near our house, had all kinds of night life, outdoor rock concerts and a very special Fete Nationale (Bastille Day)celebration.

Montreal European Flavour, huh? I guess it depends on where you come from, as a European it just looks like a NA city to me.

Managing the party is one thing and needs to be done wherever you go. However you can look at this a number of ways
1) Go to a city, rent a house in the city near public transport and visit around. This is easy in Paris or London, but you could also do Prague, Budapest, Florence. There are loads of places to do this in and I have done so in Florence and Rome.
2) Go to the country and take a country house (with pool, big kitchen and places to lounge), preferably with the odd staff (at least to arrange the odd posh meal) and to arrange day trips. Real opportunities exist in Mosel, Languedoc, Tuscany etc, Burgundy

Dugi describes the right sort of thing and certainly gites-de-france is a fantastic web site for these sort of things
2) Go to the

We use AT&T and, on our last trip to Europe, we bought a texting package.....much cheaper than calling.
We've done group trips to Paris several times and always found air conditioned apartments, sometimes 2 in one building. If that is what you choose, it is quite doable.

As nytraveler says, no city apartment has space for 12 to dine together. For a big family meal, you could either rent rooms in a hotel that has a separate private dining space, either associated with the hotel restaurant or caterable. Or plan a joint meal in a private dining room in a restaurant. Some hotels have rooms with kitchens.

When we travel with a group, everybody has to pay their share upfront at the time of their commitment. If somebody has to drop out, they lose their money if a replacement cannot be found. But this is more difficult when dealing with family members.

I agree a city offers many entertainment options. But it makes it harder to get everyone together. I think I prefer a big country villa with multiple rental cars. There you have more space, shared space, and you spend more time together as a group.

I'm sorry, but Christina is wrong again on something she hasn't done herself. I booked 5 hotel rooms for our family trip. I found a modest hotel, and everyone was a good sport about it--it was undergoing renovation, but the family who owned it was very nice and helpful to us. It only requires a credit card.
I think Montreal would be a poor subsitute, personally--like "I can't do this in Paris, so I'll just do this as a sub".
Swwitzerland is beautiful, but there is so much more to do IN Paris that is SO accessible with many many choices.
These are adult children with families. Everyone can be responsible for themselves and their belongings--it isn't a tour group where the "director" has to be the authority figure.
The one thing that will be necessary is dinner reservations when all are eating together. So advance planning is of teh essence there. We could, of course, walk into a Leon's mussel restaurant and such as that.
We picknicked on the Champs de Mars which was a good thing for a lunch. And we split up on days for ones that wanted to shop and others who wanted to do whatever. it doesn't need to be a lock step trip.

I re-read your original post and the need for a place to "eat takeaway pizza", etc. The hotel I mentioned--and most small hotels--have a breakfast room where you could probably arrange to do this.