Brooklyn Beckham and Nicola Peltz's absence from David Beckham's 50th birthday is all due to family tension ... 'cause we've learned they tried to meet up and celebrate David privately -- but, they were rebuffed.
Sources familiar with the situation tell TMZ ... Brooklyn and Nicola flew to London last week -- for the sole purpose of celebrating David.
The two reached out to the fam ahead of the big weekend, we're told .