The pictures in the book are based on the photo. Five things make a big difference to how well the child recognises themselves.
The face straight on
The most important thing is that the whole face can be seen, straight on or nearly so. A photo in profile or from a low angle gives the illustrator too little to go on.
Good light, no shadows
Daylight by a window is best. Strong sunlight gives hard shadows, and flash flattens the face.
Nothing in front of the face
Sunglasses, a cap, a dummy and hands hide what makes the child the child. Glasses the child always wears are fine – they come along.
One person per photo
Crop the photo so that only the person is visible. If several people are in the picture, the illustrator does not know who is who.
A new photo
Children change fast. A photo from the last few months gives a book that matches how the child looks now. Once you have the photo: create the book and see the result in a few minutes.