What is the source for a man not looking at a female private part - and how does this apply practically in terms of a father changing his daughters diaper / bathing her etc. is there an age limit on when this is or isnt applicable? Thank you.

The source is in the Shulchan Aruch Even HaEzer 21, which is based upon the prohibition in Bamidbar (15, 39), "and you shall not wander after your hearts and after your eyes, which lead you astray". This does not apply until a girl is able to take care of herself, so there's no problem for a father changing a baby's diaper or bathing her.