6 people taking public transportation to Manhattan sounds like a nightmare.... not to mention your cost savings will be a lot less if you can't find a metrocard vending machine in LGA since you have to spend extra $16.50 to ride the bus (no free transfer if you don't have a metrocard)

I wouldn't want to be looking for a metrocard vending machine in LGA (it's not easy) with four kids. Please just take Uber to hotel and buy your card at the nearest subway stop.