At 6-8 weeks OOP the joeys should be eating food on their own and probably do not need to be supplemented if you are feeding a nectar like staple along with fruits and vegetables each night.
Watch to see if the joeys are eating the staple in the evenings. They may not yet be eating much of the fruits and vegetables. If you are not sure they are eating the staple from a dish, you can offer it to them by a syringe or medicine dropper several times each night.
A vet check for all of them would be a good idea. You need to make sure dad is healthy and not suffering from what ever caused his mate's death. If you did not previously have them checked for parasites it is important to do that now.
If he is being aggressive with the joeys - you may need to separate them from dad but keep the joeys together to help keep each other warm.