Are these safe for nickel-allergic ears?

The maker states the stainless steel is lab-tested free of nickel, lead and cadmium, and the finish is 18k gold PVD. That suits most sensitive ears, though severe metal allergies should still test carefully.

Can kids shower or swim in them?

Yes. They are described as waterproof and sweat-proof for everyday wear, so showering and sport are fine.

The star is 4mm wide and 4mm high on a 9mm post, 20 gauge (0.8mm). They sit close to the lobe and read as small studs.

How do the backs work?

They use a flat screw-back that threads onto the post. It holds more securely than a push-back but takes a few extra turns to remove.

Are they a good first earring for a tween?

The secure screw-back and hypoallergenic materials make them a common pick for girls and tweens. The small size and fiddly closure are the main things to weigh.