Changing the tense doesn't really change the meaning significantly. The speaker thought more than one child was a girl, but was only incorrect about one.

1) You can use "turned out". They are interchangeable in most cases. I suppose the difference is regarding the time the speaker realised the child was a boy - if they realised in the past they should use "turned out", and if it is in the present they should use "turns out". 2) The speaker thought that multiple children were girls, and one of them was actually a boy.