Bus 175 is the cheapest way from the airport to the railway station.
Taxis are tricky: there are two kinds of them, the cheaper usually are from Taxi Corporations (you can see they have short telephone numbers like 9222, but someone who lives in Warsaw could tell you more, because I visit that city rarely and in my own car, so I don't use taxis), there are also more expensive "freelance" cabs, their fares can be much higher, better to avoid.
Polish railways in English:
rozklad-pkp.pl/bin/query.exe/en
(also German and French language exist)
on the image attached there's a frame (grey background, bottom of the page) it indicates the distance (471 km), and prices (129 Polish złotych for 2nd class, 180 złotych for 1st class) for this specific connection (if you choose other trains the price will be different), unfortunatelly, I don't know how old a child can be to not have to pay for the ticket.
train