how much time to plan once landing to get into Paris Gare de l'Est

Gives you 5 hours, more or less. I would think that would be enough time.
You may not want to stick with the taxi decision, because unfortunately may need 2 regular taxis, because Paris taxis are legally not required to take more than (I think it is) 4 passengers, especially with baggage.
Rick's Paris guidebook has the details on that, maybe I'm wrong about the maximum number in a taxi--but I think it might be an issue for you.
Reims is a nice destination, there's 1 or 2 days of things to do there, on your way to destinations further east or north.
Hopefully you're on the TGV to Reims and have, or will have, your advance reservations.

What about the following trains:
Option 1:
Aeroport Paris-Charles de Gaulle TGV depart 10:26, Champagne-Ardenne arrive 10:55
Champagne-Ardenne depart 11:12, Reims arrive 11:20

Option 2:
Aeroport Paris-Charles de Gaulle TGV depart 12:52, Champagne-Ardenne arrive 13:21
Champagne-Ardenne depart 13:33, Reims arrive 13:41

Saves having to go into Paris.

If going to Reims from CDG, take the 10:26 to Champagne-Ardennes then change to a local train into Reims. Takes all of 54 minutes. No taxi, no long trip into Est. With a 2 1/2 hour cushion, you could even roll the dice and buy nonrefundable Prem's tickets for only 76 euro. Only recommended for the gamblers. Otherwise buy the walk-up 230 euro tickets.

Chris and Sam have good suggestions! (since there are 6 of you)

Expect about a one-hour commute, plus the time spent lining up for a minivan (one taxi won't do for six). Gare de l'est is smaller than its neighbouring Gare du Nord and so a little easier to navigate. The blog Parisbytrain includes useful info, including illustrated tours of major stations. Here is the page for your station http://parisbytrain.com/gare-de-lest-train-station/
You can also buy your train tickets on-line, printing them out at home, to avoid a line-up at Paris Est.

Oops -- other suggestions were posted while I composed mine. Lots of options. By the way, tickets to Reims from Est , far out, are as low as 18 euros on the SNCF site (that price won't last long).

Thanks for the suggestions. I tried to buy 6 tickets from CDG to Reims through Champaign-Ardennes and it is all booked up for the time I wanted. Will more tickets come available? I want tickets for May 16th.

I doubt tickets are sold out for May (although there are often many french holidays in May) I suspect that tickets are not yet available for May 16.

Are you using www.capitainetrain.com? Tickets for May 16th are on sale right now.

Try just booking the first, reserved leg of the trip by TGV. The shorter, regional train connection doesn't need reservations and could be causing the booking issue.