It depends when they will take the subway.

Rush hour, more trains while more crowded.

Non-rush hour, less train, so it would take more time to get to Manhattan for sure.

And remember they also need to take air-train to get to the subway first. I would say it would take about 2 hours for people who are familiar with NYC public transportation system. Tourists? Probably more time.

I don't recommend to take subway if you have luggage and kids. It will be a hustle for sure.