While this post may be too late for OP, it may help others.
G7 is a reliable, large taxi company in Paris. One can book a cab for 5 (or more) people through them:
http://www.taxisg7.com/services/large-taxi-monospace
Cost from CDG to the Goeblins should be around 60 to 75 euros-that G7 site has a cost estimator one can use. By comparison, the cost to use the RER/Metro is around 10€ each, so the family of 5 would be around 50 euros.
The savings by not taking a cab are hardly worth it. That plan to change from RER to Metro at Chatalet looks good until you do it, especially with kids and luggage. Confusing corridors, stairs, escalators (if they're working), long walks from one platform to next. Plus walk to apartment at Goeblins. Really penny wise, pound foolish. Skip a t-shirt and take the cab.
Vans are ok, when they show up and you can find them at CDG. Read up as this is often an issue. I am not sure a van saves much over a cab for 5 people.