Phoenix - we once went to a Thomas Day at NRM by accident (was just going generally). You will enjoy it, so don't let me put you off... but feel I should warn you, it was HEAVING. Make sure you get there early if parking!
Personally, I'd rather go to Thomas Day at somewhere where the rides are longer (and less queuing) eg Keighley and Worth Valley (though of course it is still busy, just not quite as much), and just enjoy NRM separately for what it is....
Def don't admit to 2 btw. I once did on a tiny train on DS 2nd bday and they charged me anyway. Tight b8stards!