What to expect in terms of accommodation price - Family of 6 (4 adults - 2 children 7&11)

That is nearly impossible to forecast - too many variables. If using standard hotels/B&Bs you will need what is commonly called a family room for four and another double. Ball park - maybe 400 to 600 euro/ night. An apartment probably would work better for you but no idea on the price.

If you're looking for apartments in Paris, you'll have to decide what kind of sleeping arrangements you can tolerate, as well as how many full bathrooms will be included. Expect the apartment to be smaller than you would like, and it might not have an elevator and will not have air-conditioning.
The price range for a family of 6 would mean a deluxe apartment, and it depends on how far in advance you could book one, because they are normally booked up a year in advance.

If you want 2 or 3 separate bedrooms, each with "real beds" and a full bathroom, the price range will be around 2000 to 2500 EU per week - possibly more. If you are staying less than a week, prices will be higher.

Damage deposits and cleaning charges might be included or listed separately.

If staying less than a week, you might try finding adjoining rooms in an apart'hotel, such as Citadines or Adagio.
Le Roi de Sicile and Hotel Residence Henri IV are also nice apart'hotels.
All of these locations have very good transportation options.

AirBnB and vacation rentals in general will vary. You need to check the actual address using GoogleMaps Street View to be certain that you're comfortable in the neighborhood.

No idea what the other situations would be in the cities you mentioned, but I'd expect them to be even more expensive than Paris is.

The only hotel I personally know in Paris that offers a family suite for 6 persons is the Hotel Latin Excelsior. It's a nice hotel, and you can see their prices on their own website or email/telephone to inquire.

Thank you all for the reply. More than a forecast i am looking at past experiences of people who have traveled as a family or know about it and what is the approx cost to get an idea/set an expectation. This is the first time i am travelling with such a large group and hence the question.... I understand these destinations are pricey.

What is it exactly that you want to know?
Prices of groceries, transport, admission fees to attractions?
It's highly doubtful that many families of 4 adults and 2 kids could chime in with any suggestions...

In a hotel you would need one double room for 2 adults and one family room for the parents with kids.

It is very important to note that hotels in Europe usually don't let kids older than babies stay free - the rooms don't usually have 2 double beds etc - so you need to specifically book a room for 4. So 2 rooms, one a family room - several hundred a night total. $400?

A 2 bedroom Airbnb might work, if the sofa folds into a bed. A 3 bedroom would be easier. $200-300 maybe

I am assuming 2 couples, one of which is the parent of the kids. If each adult needs their own room it will be much higher!

I travel with kids - elementary school - and in most places can find 2bedroom
Airbnb or other apartments for under 200/night but you have 4 adults which changes things.

Thank You Mira. This was what I was looking for. This helps a lot and I know what to search for now.

We don't mind a 1 or 2 bedroom apartment with comfortable sofa beds for the kids. The kids aren't too big (7 & 11) and don't mind sharing the bed with each other or with the parents.

From what I searched I see approx. 200 - 250 EUR a night for 6 people, either an apartment or 2 triple rooms or a family room + double per night, for a place we think is comfortable and reasonable located to travel around. So I need to keep my rpice range around that figure or higher!

Thanks!

A typical Quad room runs at least 250 Euro; a double on top of that another 150 although it is fairly easy to find small 2 star hotels in Paris that charge about 100 Euro for a double; they will be in outer arrondissements rather than in the center. Take a look at something like booking.com with your needs -- 2 rooms and 6 people to get an idea of some options and costs. The cheapest central hotel I know the Marignan has a quad for 150-170 which is the lowest I have seen. It is usually booked up for this reason so if it interests you book early. It is a bare bones budget hotel but well located.