First of all, "Estar" is used to express: 1. geographic or physical location: for exemple: ¿Dónde estás? Estoy en el laboratorio. ¿Dónde está Chile? Chile está en América del Sur. 2. state or condition 3. many idiomatic expressions 4. progressive tenses We can classify your sentence under the first case so that the question will be: Donde estan mis pantalones? Sus pantalones estan aqui "OR" Aqui estan sus pantalones While "Ser" is used to express: 1. the hour, day, and date 2. place of origin 3. occupation 4. nationality 5. religious or political affiliation 6. the material something is made of 7. possession 8. relationship of one person to another 9. certain impersonal expressions 10. where an event is taking place 11. essential qualities

I agree with Manel: - "Aquí están sus pantalones" (Your/their/his/her pants are here). In this case "estar" is used to express psychical location. - "Éstos son sus pantalones" (These are your/their/his/her pants). In this case "ser" is used to express possession. :)

Aquí están sus pantalones. It's perfect. Aquí son sus pantalones. It's bad.