If all three of your children are over 11 years old, then the total RER 'B' & 'A-4' fare will be 30,50 €uro (adult fare-6,10 €--half-fare for children under 11 is 3,05 €).
RER 'B' from Gare du Nord [in the directios of Massy Palaiseau ('K'xxx'; Saint Remy Les Chevreuse ('Pxxx' or Robinson ('Sxxx')] one(1) quick stop to CHATELET-Les HALLES, then make the quick train transfer to the RER 'A-4' ligne (heading in the direction of Marne La Vallee-Chessy--any train with a 'Q xxx' designator) to the end of the
A-4' ligne at MARNE La VALLEE-CHESSY station. The route and the transfer between RER lignes isn't really difficult. complicated or hard to follow....but the question is: Do you want to make your way through the RER system, dragging that large a 'tail' behind you??
I'm not sure what the TAXI PARISIEN fare would be (perhaps 55-70 € ??) But with five(5) passengers, you would require a mini-van type taxi and have to pay the official surcharges of 2,70€ per person for the 4th & 5th passengers and ,90 € for eack bag---this in addition to the metered fare out to Zone 'C'.
The various Paris airport shuttle service will also provide transfer from Paris train stations out to DLP...but the charge is likely to rune anywhere from 80-120 €uro.
I guess you missed my post from yesterday: