Two options
Cheaper option
Take two cabs with one adult and one child and half the bags in each cab
or try to squeeze into one cab that is a mini-SUV (depends on the amount of luggage)
Two normal yellow cabs would costs $55 x 2 = $110 (inclusive of tips and tolls)


Book an SUV with 3 rows of seats (6 seats) and loads of room for luggage. I used Dial7, which is bookable over the internet, is realiable (used them from both EWR and JFK), driver meets as you leave customs area with you name on the a sign. Down-side is that after tips and tolls, this will cost about $190-$200 but it keeps the family together.

A stretch limo will cost the same but only takes a limited amount of luggage (but will give the kids a thrill)