from Charles de Gualle to Paris on crutches

I think most people would suggest a taxi, I can see that a typical taxi might not fit your whole family. I am not sure if larger taxis are available or if you would need to take two. I am sure that there are people on this forum that would know or might even have a suggestion for a private driver with a larger vehicle.

Here is some info about taxis.
http://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/taxi/paris-cdg-taxi

Here is one website I found about private transfers. I have no personal knowledge about this company

https://www.blacklane.com/en/airport-transfer-paris

Only one answer here a taxi; taxis have a fixed price for travel from the airports to Paris and can take up to 7 people. For a large group the taxi is little more than the train where you go up and down stairs with your luggage and are vulnerable to pick pockets with all your stuff. A taxi from CDG to Montmartre for 6 people and their luggage will cost 58 Euro -- 50 fixed fee plus 4 for each of the 5th and 6th persons. No tip is needed. A couple of Euro if the driver is very helpful with luggage and your special needs is appropriate. Be sure to use the official queue and let the dispatcher know how many you are; he will match you with an appropriate size cab. Have your address written down. Anyone who approaches you asking if you need a taxi (and the crutches will be a crook magnet) is a scammer. Someone in this or another travel forum reported being charged over 200 Euro from such a scammer.

Montmartre is a fairly terrible location for someone with mobility issues; it is very hilly with lots of pathways involving very steep stairs to get from one street to another. It is a terrible place to negotiate with crutches. It is also fairly far out and away from most of the places a family would want to go on a first trip. I love Montmartre and have stayed there for a month at a time, but I don't use crutches and I am not going to the central tourist sites every day.

The metro is also not suitable for people on crutches as it almost always involves steep stairs and from Montmartre you would be changing trains to get most anywhere which ALWAYS involves flights of stairs on the metro change. You would want to use buses and your husband would be entitled to a seat. But you are setting yourself up for a lot of commuting and hassle by staying so far out of the center. Do look into getting every one a Navigo Decouverte weekly transport pass if your dates fall mostly within one M-S week. (they always run Monday to Sunday.) That way you can hop the bus for short distances without thinking about cost or tickets.

Thank you for your help.

I'd wait for a taxi that can take all of you. It's called a "break" or mini-van. The agent will put you in one of them. No luggage fees.
Montmartre is trying at the best of times. You can use the little blue train to get around, though.
It's difficult for taxis to get through - you will spend a fortune if you want to go anywhere.

I wouldn't bother with an expensive and undignified tourist train, the RATP runs a Montmartrobus circular service with tiny little electric buses that uses normal Paris bus tickets.

Here are a couple of links to information about the Montmartrobus that may be helpful. It is actually fun to ride. I used to drive a bus many years ago, and it is instructive to watch how nimbly the drivers maneuver their vehicles through those steep, narrow, and twisty streets.

https://francetravelplanner.com/go/paris/trans/ratp/bus/montmartrobus.html

http://www.eutouring.com/paris_montmartrobus_map.html

Parisian buses might be superior to the metro, since transfers would not involve more than a few up and down steps.