Arriving at CDG at 5PM on a Friday. We normally would take a taxi to the hotel off Blvd. St. Germain. At that time of day, however, is there likely to be a lot of traffic that will slow the taxi trip? Would the RER line B to St Michel/Notre Dame be faster? The two arriving are young and carrying only backpacks, so they might walk from the station to the hotel, or take the subway to St. Germain station, or uber. I just want to get them to the hotel as quickly as possible. Another option is to have them go straight to the Louvre, and meet us there (plan is to see the museum during its late hours). If taking the RER, would they get off at Chatelet and walk from there?

The train from CDG into Paris is very simple and efficient. It is the only way I come into/out of Paris when I visit there. I just took it this past January and the price beats a taxi any day! Bon voyage!

Yes, it will be rush hour. Starting at that time.Yes RER will be faster, as it is about as fast when there is no rush hour.
Young and backpackers is a nobrainer : take RER : easy to find, cheap (for 2 it will be 25 €, half price of a taxi) and fast.
Chatelet is a labyrinth. I'd say to stop at St Michel, easier to find an exit and to get their bearings.
No experience with Uber - I'm French (and old).

Definitely the RER B to Saint Michel-Notre Dame. They can walk to boulevard Saint Germain from there or else take metro line 4 one station to Odeon, if that gets them closer to where they are going.

I agree, I can't imagine why they can't take the RER which would be fastest.

Of course if they need to get to St Germain metro stop, they would get out at St Michel. There is a metro line from St Michel to St Germain stop, you know, I can't imagine why they would need to take Uber, which involves a lot more trouble and expense (such as getting out of the station, just for starters, not to mention and waiting for it, if it even works there). Even that would take longer than just taking the metro a couple stops.

Thanks all.....I will recommend the RER B. If they were going directly to the Louvre, which stop would you recommend?

If they were going directly to the Louvre, they could easily walk there from Ch telet-Les Halles, but the actual museum entrance is at Palais Royal-Musée du Louvre, so a transfer to metro line 1 for 2 stops.

Keep your ticket until you are out of the system- sometimes at exit gates you need to put it in to get out of system so keep it handy - at transfer stations you may have to put it in machines again and to finally get out of system may need it too.

Not sure if still so but some RER trains from CDG go 'sans arret' to Paris Gare du Nord - as folks can only board at CDG it may not get as crowded as the omnibus trains that stop at every station.

I agree with PalenQ's suggestion for a non-stop, but it doesn't look like there is one available for their arrival time. paris_rer_b_train_schedule_south_to_paris_2018.pdf

I assume the 5:00 reference in OP is landing/arrival time. If so, and they are coming from out of country, they must go through customs. No luggage helps time, but the best time they could hope for would be to arrive at Chatelet or St. Michel right around 6:30 p.m. They will have to go to check their packs if they are larger than 21"x13"x8" under the Pyramid. They are looking at, best case scenario, getting 2-2.5 hours total in the museum and that assumes everything works perfect.

Will Louvre store their backpacks.. as I know they will not be allowed to bring them in the museum.

From the Louvre website:

Cloakroom

Self-service lockers are available free of charge beneath the Pyramid.

PLEASE NOTE: any items exceeding 55 cm x 35 cm x 20 cm are not permitted in the museum.

All items left in lockers must be collected the same day. The Musée du Louvre is not responsible for lost or stolen property.

I agree with everyone that the RER is the best option for this scenario, but just wanted to chime in that Uber DOES work in Paris - quite well in fact!
Harder to get at the actual airport, but very easy to use in the city center and surrounding areas. Just for future reference!

Oh I just love the term, the "cloakroom".......

Are you frenchspeaking Alice ? Cloaque indeed means either a sewer, a kind of anus for those species that don't separate liquid and fecal matters or just simply a place that stinks and is full of disgusting (mostly liquid) tihings.
In French we say 'Vestiaire' for the place where you put your 'vest' and other belongings and yes there are cloakrooms in every museum.

Tell the kids to be alert and watch their backpacks on the RER. I turned mine around and wore it in front. Some zippers unzipped but nothing lost.