cheapest/easiest way to get to hotel from schipol

I don't think you could fit six into a taxi. Another option is the Connexxion hotel shuttle: http://www.airporthotelshuttle.nl With kids in tow, and the late time your are arriving, this would be the most stress-free way to do it. http://wikitravel.org/en/Amsterdam#Get_in

In the middle of the airport is a Tourist Information center. Someone there can help you. I can't find my guidebook but maybe some hints might help. We stay at the Beethoven Hotel, just on the edge of the museum district. From the arrivals floor and the TI you will go downstairs to the immigration gate, get you passports looked at, and you will turn right towards all the train stances, ticket kiosks and the ticket counters. We use the counters and buy tickets to Sud (south) Station, get on the train and go to Sud. Sud is a very small station and we go into the ticket counter and buy a ticket for no. 5 tram. Then having asked where to catch it, we get on and get off at the third stop. No. 5 goes all the way to Central Station. If you use the tram you will have to stick your ticket in the little slot by the inside of the door. The ticket counter person tells us how to do that. This would be the least expensive.

Unfortunately the TI office will be closed by 9pm and the shuttle bus I mentioned above also doesn't operate past 9pm, so it looks like you'll have to use train/tram. The trams don't use paper tickets/slot machine anymore. You'll be given a disposable electronic card which you place in front of a fancy card reader.

does anyone know how much the train to central station is? i cant seem to find that anywhere

http://wikitravel.org/en/Amsterdam#Get_in Tickets purchased at the ticket window costs 4.30 euros one-way.

If the TI counter closes at 9:00PM I take back everything I said.