No. No chance.
If you have the money to spare for cab for 6 hours, you're better off taking a cab from EWR to Liberty Science Center and have the cab wait for you while you visit. IIRC there is a decent view of Miss Liberty from LSC. Its a great view of NYC and a great place for kids regardless. But, with that little time, you would need to hire a car for the duration to/from the airport