Attempting to find the first index of a group of nodes

Hello,

I have a node with several child elements of different types. I would like
to find the first index of one of these child elements. For example I have 

<parent>
   <child1 />
   <child2 />
   <child3 />
   <child2 />
   <child1 />
   <child4 />
   <child2 />
   <child3 />
   <child5 />
</parent>

I would like to set a variable that is equal to the position of the first
appearance of <child 4/> or <child5/>. In this example it would be <child4/>
at position 6.

Thank you,

Joe Melton

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