Sure, here are some vocabulary words that start with x that are suitable for kids:

Xylophone: A musical instrument with wooden bars of different lengths struck with mallets to produce musical tones.

X-ray: A type of electromagnetic radiation used for medical imaging to see inside the body.

Xenon: A colourless, odourless noble gas found in trace amounts in the Earth's atmosphere.

X marks the spot: A phrase that indicates a specific location, often associated with treasure maps.

X-ray fish: A type of fish with translucent skin, allowing its internal organs to be visible.

Xanadu: A place of great beauty, luxury, and contentment, often used to describe an idealized or fictional utopia.

Xerox: A brand name often used as a verb to refer to making a photocopy.

X-factor: A variable in a given situation that could have the most significant impact on the outcome.

Xylography: The art or process of printing from woodcuts or engraved plates.

Xerophyte: A plant adapted to survive in dry conditions.

Introducing new words through activities, games, or storytelling can make the learning process more engaging for kids. Feel free to create fun and interactive ways to help them remember and use these words in context!