Is Someone Stealing My WiFi?

I recently got a decent internet service, but I've noticed some kids hanging out near my house. We live in an alley, so it's not too unusual, but my parents are worried they might be stealing our WiFi. I've taken some steps to secure it, like stopping the SSID broadcast, changing the WiFi password, enabling a MAC address whitelist, and turning off WPS. After checking, it looks like only our devices are connected. Even so, these kids are still around. Is there any other way they could be accessing our WiFi? What more can I do to secure it?