If you do not have much luggage, take the AirTrain to Jamaica station and LIRR train from there to Penn Station. Penn Station is located on 7th Ave.
However, if you are nine, it makes sense just to take three cabs. Each cab will cost you about $50, while the train will cost you about $12-13 a person.
Subway, if you have luggage, is not recommended. Will take you about 1.5 hrs, and for non-New Yorkers it is very confusing. It is the cheapest option, but still costs you $7 a person($5 for AirTrain and $2 subway fee).
good luck
If you do not have much