All of what has been mentioned here is useful.

Two more possibilities:

1) If the recorders are consistently a half-tone flat, try blowing a bit harder to see if they will "come up" to pitch.

2) "Cold" recorders (below exhalation temperature) are often a bit flat until they "warm up" to pitch; playing the instruments for 15-20 minutes and then rechecking their pitch will reveal this.

Another difference between recorders and whistles is that the recorder has 7 holes in front (8 with the octave thumbhole) and the whistle has only 6 holes in front (and no octave thumbhole).

Just my $0.02, fwiw.

Good luck!

V (who tootles on both whistles and recorders)