Is the chest x-ray 2 times in a week is safe for my 7 yrs old child?

WHY?: Depends. If there is an important reason why a 7 yr old needs that frequent an x-ray evaluation and not getting the x-ray resulted in harm then the x-rays would be safer than not getting them. Otherwise, the small but not zero risk of exposure cannot be justified. Why would this need to be done and for how long?

Unlikely: Unless the equipment is ancient (like the 1950's) before advances made the radiation exposures for these studies a fraction of the old ways, there is little to be concerned about.