Xylophone: fun, colorful, easy for a kid to remember as a cute little instrument
Xenon: An inert gas used in... MRI scans, I think?
X-ray is probably the only other 'X' word with more real-world representation than xylophone, and as pointed out above, that's not quite representative of how the letter is used phonetically in the rest of the language.
Tortoise was the first group i heard using them, led me get into some of vibraphonist Cal Tjader's material and some lesser known vibraphone groups/artists (though Tortoise is much more my speed) https://m.youtube.com/watch?v=EPqmgY6WTVw
Though she often played marimba, Frank Zappa's percussionist Ruth Underwood was a monster with the mallets as well... Again not vibraphones in this particular example but anyone who digs them may want to see this: https://m.youtube.com/watch?v=e7Sq0chFjps
As a father of toddlers learning ABCs, I agree. I've seen some weird side steps, like X-ray Fish for an animal themed one, or Xerox--- a company name. Or just straight up Fox because they couldn't think of Xylophone or any other X words.