What you are currently doing sounds a lot like what many call co-parenting. Basically this means leaving the babies with mom (and dad) but taking them out to socialize them and supplement their feeding. Most of what I have read/heard on this method makes me think it is the best way to raise the babies (JMHO). In this instance it teaches birds to interact with their own species and see how to both fear and like humans. Not that it is a good thing to fear us LOL but it does help to have the instinctual mistrust of larger beings which could hurt them while still learning to accept them on individual basis. (Not sure if I am explaining this well).

As long as mom and dad aren't hurting the babies in any way I would leave them with their parents even if they aren't being fed enough. It will help teach them to eat real foods by seeing it being eaten by their parents when the time comes. Every 3-4 hours sounds fine just make sure their crop is not being overfilled and that it is allowed to "completely" empty at least once during the day or night to protect against sour crop. If the crop seems to be staying partially or completely full then withhold feeding because either mom and dad are doing their job or the food isn't being digested.