Student or Learner
Can I say "when I were a child I liked to skate"?
Is it more polite than saying "when I was a child I liked to skate"?
I'm thinking of the conditionals where you can say "If I were a child, I'd like to skate."
The indicative form was is used in the when-clause, because the speaker actually was a child.
The subjunctive form were is used in the if-clause, because the speaker was not a child when s/he uttered the words. This is a hypothetical, counterfactual utterance.
As BC wrote, the use of were in your sentence is incorrect.
In the if-clause, were is correct, though many people, especially speakers of British English, use was these days.