It's not impeding a child's' right to pursue happiness because we have laws to protect those rights just as the 1st amendment strongly states "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press, or the right of the people[...]" Which has been interpreted multiple times to extend freedom of speech to freedom of expression. There is nothing standing in the way of the child going to court to officially change their name anytime they wish to do so. The parents were merely exercising their rights given to them under the 1st Amendment.
Doesn't mean we have to agree with it but who are we to judge others actions? We allow offensive tattoos, haircuts, piercings, etc in our career alone... so why can't we allow this pass with out judgment?