FairyGirl, do you have lineages on your breeding pair? I ask because nature has its ways, and sometimes the reason why glider parents kill their joeys is because they sense something inherently wrong with the joeys, not only inexperience. If you don't have lineages, it is possible that the joeys are somewhat inbred and have defects we don't notice.
My recommendation would be to neuter dad if that is the case. The experience of losing joeys is not bad only for us, but also stressing for the parents. If you have lineages, then you could go ahead and try again, many times first time parents can't handle raising joeys the first time, but do better later on; but there are also cases of parents that simply are not willing to raise joeys, if it continues happening it may be better to simply neuter dad.