One of the options is to park at Metropark train station and take NJ transit to EWR. It has ample covered parking ($9/day) which would translate to about $72/8 days for parking and $11.25/person for train tickets. You could drop off your family at EWR and then park and take the train to EWR. No need for hotel or park/fly packages. Plus it is a very safe area and you won't have to worry about security!

You will have to build an extra hour for doing all of the above but savings will be tremendous.