Scrambled or hard boiled egg would work as well, no butter or oils.
Personally if it were me, and please know that I have never had a female have Joeys so I am not personally knowledgable, but again, if it were my gliders I would watch carefully and see if that is the only time he is showing aggression, if so, then I would feed her seperately. If you are noticing aggression elsewhere and you're concerned for the little girls safety but she can't leave mama, I would seperate him until you can decide what to do, ie; get him neutered or possibly wait til she's big enough to fend for herself & try putting them back together, or maybe having to make the decision to move her to her own cage. Fact is, you'll have to do one of the above at some point anyway because he will breed with her when she is old enough.