Can a precum make a girl pregnant if we both using pants?

Probably not: If you did not have any intercourse or genital contact, it would be unlikely to cause pregnancy. Do remember though that anytime you have unprotected sex, there is a possibility of pregnancy. Always use condoms for std protection and an effective birth control method (other than just condoms) when you have sex, if being a parent is not something you want at this time.

Unlikely: Because the clothing acts as a barrier.