Kids all have an underlying and incredibly powerful drive to grow, develop, and learn. They will use whatever brain tissue they have and do as much as possible with it. All of us know kids with severe progressive neurodegenerative diseases who for years went undiagnosed because they were making as much progress as they did despite it for so long. And we all know that even kids with Hydranencephaly, having just a little basal and gangliar tissue, will use it to connect with and recognize caregivers, enjoy some music, etc...I guess they save what they've got for the truly important things in life!!
A bad looking CT or MRI is just a picture of an injured brain, and does not in itself tell you exactly how much that brain can still do, or predict lifespan.The other, related part of this picture is "plasticity". The younger the brain the less committed certain parts are to doing certain specialized functions. But plasticity is not infinite, and it is not a free lunch.
So when there's not a lot of white matter, what there is works a little overtime to get through what messages it can. When not enough messages that control excess tone are getting through (when it comes to motor functions especially, the cortex is largely inhibitory in function), we see a lot of spasticity, and we try to use medications and such to send the message to relax. Then whatever messages to move a certain part a certain way can get through will better be able to accomplish something. White matter also has some function in learning, connecting the different areas of grey matter together and connecting the hemispheres with each other. The "diffuse axonal injury" seen after head trauma is largely a white matter injury, disrupting lots of connections, and this generally results in an overall inefficiency in the operation of the brain. Exactly how new connections get established and how brain function continues to improve over months and years afterwards, is still more than a little mysterious.
Remember that the resolution of even the best neuroimaging is still a little limited. There are many things we could not see on CT scans that we can see now on MRIs, but you can't see down to the level of an individual nerve cell by any means.
So remember that every brain is designed to grow and develop, and even an injured brain will grow and develop to whatever extent is possible. Your child's abilities and potential to maximize them matter more that the picture of how the brain looks. If someone is diagnosed with mental retardation (about 50% of people with CP have some degree of MR) it does not mean they cannot learn. It means that they may learn more slowly, require more repetitions, may have to be more explicitly taught things that others just learn "naturally".
We all face the challenge of pointing out that the cup is 3/4 full instead of empty- there is a world out there that tends to see it as either 100% full or nothing.