There are very few 'perfect' books out there for children with CVIs, and even fewer when you start considering the individual preferences of each child.
Modifying books may therefore be something that you would like to do, either with just a quick tweak here and there, or with a much more involved approach.
I would therefore like to start by stating clearly and categorically that nobody expects you to do this! You have a complex child. You already work harder than is humanly possible. The hope for this site is to help drive change, and get publishers doing this for us. So please please please don't let this become yet another source of guilt of something you 'should' be doing but really can't.
If, however, you quite enjoy arty endeavors and would see this as therapeutic, or have a friend or family member that you could rope in to do this for you, then I hope this overview might be of some use.
Increase the contrast and clarity of the main image
Improve image outlines with a black marker pen
This is my favourite method as it requires very little artistic talent and next to no concentration. Yes, it can still be very time-consuming, but you can do a few minutes here and a few minutes there, without making any mess that needs clearing up.
Examples of using a black marker pen to improve outlines
(Pop-up Dear Zoo, Rod Campbell and Maisy's Bedtime, Lucy Cousins)
With paperbacks, you'll need to do a quick test to make sure that the ink isn't going to be visible on the reverse side of the page. Generally, however, this method seems to work just fine as young children's paperbacks tend to be made of thick, glossy paper.
Simplifying and highlighting individual images
You can simplify individual images (e.g. flowery dresses, patterned curtains etc) by covering them over in a block colour of cardboard.
Where that image is a key part of the story, it's a really nice trick to use tactile raised materials, shiny card, or anything else that would increase visual interest and attract the child's attention to that part of the illustration.
Depending on the severity of your child's CVI, by increasing the visual clarity and interest of the subject of the illustration, you might be able to avoid having to deal with slightly too busy backgrounds altogether.
Examples of simplifying and highlighting individual images
(Maisy's Bedtime and A Busy Day for Birds, both by Lucy Cousins)
Maisy's book has been covered over with a square cut from a novelty shiny green jiffy bag
The snake has been simplified and highlighted by sticking on some rolled up aluminium foil
Blue card with a large felt sun has been stuck over the original, too busy page.
Get rid of the backgrounds
Paint them out
Not something I have really tried, but others have reported success with black acrylic paint. See here for some examples by 'Emma Bear' author, Lauren Seeger.
As far as I can work out, this only works with black, as when I briefly experimented with other acrylic colours, it would have taken multiple coats to cover up the backgrounds.
Use cut-outs (e.g. cardboard or felt)
Obviously you could do this a lot better than I have if you have the time/inclination/artistic skill, but hopefully this gives you the general idea!
Example of using thin black cardboard as a cut-out
(A Busy Day for Birds, Lucy Cousins)
Transfer the image onto a plain background
You can achieve this either by colour photocopying the image (enlarging it at the same time if desired) and cutting it out, or by cutting out the image directly from the book itself if you are brave enough to do so.
If you've decided to cut-out the images from the book, and the picture pages back onto each other, then you'll either need two copies of the book, or you'll need to edit the story by just using selected pictures.
These cut-outs can then be stuck onto a plain background of your choice. For longevity, you could laminate your new sheets (be sure to use matt laminating pouches as the glare from glossy surfaces can be very distracting). You can then attach your sheets together, or just use as individual pages.
You can also buy ready-bound blank books, such as this one.
Example of re-mounting images onto a plain background.
The characters have been colour photocopied (and enlarged), then cut-out and mounted onto plain backgrounds (A4 size). The sheets have also been laminated using matt pouches.
(Jungle Friends, Little Tiger Press/Rachel Baines)