We did the long term/economy parking thing last year, and have to say we probably wouldn't do it again. At least not in January and not with a young child. Yes, the shuttle did come quickly, and we did save some money, but we had no prior knowledge that the parking was so far away (our bad), and just made our flight. When we got back to Newark, I stayed at the baggage claim (at 11 pm) with DS (8 mos.) and all our luggage alone while DH went to get the car. We thought this would be easier, but it seemed like an eternity and have to say, I was a bit nervous.
We got a great price on our flights for next week's trip, so we decided to get a car service with the savings (not to mention the savings by using the Magical Express) - maybe this is an option you can consider, too.