We are staying very close to the Kronenburg tram stop (Amstelveen) and we need to be at the airport around 06:00 for an international flight. Would it be possible to catch a bus that early? And if so, where?
On a normal weekday this schedule will work.
Leave the Metro/tram stop at 5:29, metro 51, to metro/tram stop Oranjebaan, walk to bus stop Oranjebaan and take bus 300 to Schiphol. You should arrive just before 6 AM.
From this time of day onward there are regular metro's, trams and buses, overal travel times run from 22 to 35 minutes. (About 4 different connections will get you at Schiphol before 6:30.) Earlier connections are by night bus and run about once per hour.
Weekends you are longer dependent on night buses. But those on Sunday morning seem to run every 15 minutes or so and take the same amount of time.
If there are several of you, look into a taxi, that might work out cheap enough. Pre-booked taxis are also available.