Is it permissible to avoid childbirth to prevent future hereditary disabilities and to devote oneself to caring for currently disabled children?

Permanent prevention of conception is forbidden except in rare cases, and the possibility of a fetal disability is not one of them. It is advised to put trust in Allah and supplicate, and to be content with His decree and destiny, for all of Allah's decrees are good for the believer. What befell the two children is a trial and a test, so you must be patient and seek reward from Allah, and not show displeasure with Allah's decree.