What is my sons relationship to a) my first cousin and b) to her children. And what is my sons relationship to my nieces children.No they aren't the same. First thing you should note is that you have different generations so there will be some times removed (as in 2nd cousin 3 times removed). Times removed refers to the number of generations between each person.
I keep coming up with the same answer to each of the questions but surely they cant be the same??
Then you have cousins versus nieces so right away you have different relationships.
For example the child of your first cousin is your son's second cousin.
Your first cousin is your son's first cousin once removed
You could try Steve Morse's Relationship Calculator. Read his instructions and start entering your terms. Also try this Relationship chart as it may be easier to use.