Find a nutral area like the bathtub or a sofa that no rats have been on and put them all down to play. Try to have someone else with you so if any problems arise like a fight it can be quickly broken up. If there is no puffed fur, fur flying literly, hissing, blood, or gashs keep letting them play. As long as you feel safe the babies can stick up for themselfs against a older rat and they are getting along well you can take the next step.
Clean the cage really good try a little bit of bleach with hot water then put the babies in for 15 min after that set the older rat in.
Once again watch for any problems and supervise for a week or so after.
Force grooming, some fighting, some squeaking is ok.