That was pretty much the same with us Scorpio, DS was only just saying three or four words when he met DP and would call all men 'dada', we tried for ages to correct him to say Johnny but he wouldn't, then after a few months of it DD asked if she could call him Daddy too, so they both call him Daddy.
ds calls his real dad 'Mick' and apparently when he is with them they say to call him Daddy but he won't, and DD said she ahs slipped up a few times, calling him Mick and refering to DP as Daddy.
The way I see it, XP could've been the man they looked at as Daddy but he threw it all away, it's not my fault that DP was perfectly happy to be their Daddy and they were perfectly happy to accept it, it's his.