Charles de Gaulle to Le Halles with Family

A taxi from CDG for four people will be 50 euros, hardly any more than the tickets for the RER (and way fewer steps).

Taxi fares are fixed between the city of Paris and the airports.

Since Les Halles is on the Right Bank, a taxi there from CDG will have a fixed rate of €50, and no tipping is required beyond this. The RER is €10.30 per person; I believe the infant would ride free.

I don't know what you consider "easy," but I know that for me, wrangling an infant and luggage on the RER, while jet lagged, would not qualify. I'd spring for the taxi; it's a low price to pay for ease of travel.

Note that I take the RER all the time. But, I'm traveling solo, and I've been to Paris multiple times before.

I would not do the RER as a family of four with a 20 month old. It gets really crowded. There is no luggage storage, so as it gets more crowded, the more your luggage is in the way. We do it as 2 people with backpacks only, but wouldn't do it if we had more.

After a long flight, just take an official cab from the cab stand (not from someone who comes up to you) and pay the 50 euro. Easy.

Thank you
The main problem with a car is that we will need a car seat. I got one quote for 90 euros for van with car seat. Any suggestions for this?

There is no requirement for car seats for children in taxis. I would simply bring the child seat I used to reach the airport at your originating point of departure.

I have four kids, and we were diligent with car seats. But, sometimes it just doesn't work. You wouldn't have a car seat on the train, either. I would buckle the youngsters in regular car seats. Another option would be a booster seat on the plane, use it in Paris, and leave it for some young family.

The RER B originates at CDG Terminal 2, so you will be the first people on the train able to get seats together and room for luggage. Les Halles station is a bit of a maze. I haven't been in it since it was rebuilt a couple of years ago, but there is an integrated underground shopping mall in it. Here is 2-D underground map, Sortie No 2 will be close to the Novotel across the street on Rue Berger.
https://external-preview.redd.it/ct1UrjUq4v1FD9Gm-VDDKs43A85HHoqE1sfQl8eGGiw.jpg?auto=webp&s=0884f274603439e05ad69406b179c7c02a1cc1ba

Even though car seats are not required in taxis, when I had a toddler, I used to travel with my own because I have a different risk/reward ratio when it comes to my children than officials setting regulations. If you do not have a car seat but want to drive from the airport, book a taxi with a car seat from G7. Really though, you are staying in an ideal spot for taking the train. It will be a tiny bit over 30 euros (the toddler is free) and you do not have to make any connections. The hotel is extremely close to the station and there are escalators and elevators within the station. If I did not have a car seat and I was traveling with a toddler, I'd take the train in your situation.

Edit: E-mail the hotel and ask them which exit to use from Chatelet-les-Halles for easiest access to their entrance. I think the best exit for you is Sortie 1 (Porte Marguerite de Navarre) in the Forum Sector but the hotel can confirm (or not) that.

Here’s the response I got from the hotel.
It seems like if we got on the train st the first stop we should have space to work with and there are elevators and escalators to exit at our stop.

Dear,

Chatelet station is huge. However it’s quite easy to locate yourself, there a lot of sign.

You’ll have to take the exit number 1 called Marguerite de Navarre.

You’ll have escalators and elevators that will lead you in front of the hotel.

Sincerely.