🚀 go-pugleaf

Hmmm. Tough to advise without actually seeing the logos,etc. of the brands in question. From a conceptual standpoint, I'd suggest looking at the brands and seeing if there are any elements that they have even remotely in common. Even something as slight as a similar font, shape or color may be something you can seize on. The easiest way to bring cohesion between the brands might be to adopt the color palette of the parent company and apply it somehow to the existing materials of the brands in question. If that's too radical for your boss, maybe you could produce a new element, derived from the materials of the parent company, that could be added to each of the 5 brands. Perhaps a small symbol taken from or reminiscent of the parent logo that could be worked in subtly. In general, if you can't really change what's there your only real option is to add something to it.

Your employer surely must understand that what's being asked of you will necessarily require that the individual branding of each will need to be altered. After all, you can't "revamp" something and have it stay the same. I would suggest that in order for the images of these individual companies to effectively boost the esteem of the whole organization they need to be mutually supporting. By this I mean that the image and branding of each should suggest that they all "play for the same team". This may involve a more intense overhaul, but may ultimately provide a more powerful overall image for the parent company.

These are just general suggestions for your approach to the project. How well they apply to the task at hand depends on the specifics, which I have no way of knowing. I'm sure it's easier to merge the logos of financial institutions than, say, soft drink brands. Anyway, I hope it helps your thought process. Good luck!