Suggested wording:

"If you select a children's book for review that is meant to be read by very young children (i.e. children under 10) or meant to be read by a parent/adult to young children, it is allowed and recommended (but not required) that you either have your child read it and/or that you read it to your child. Otherwise, it might like a bald man reviewing a hair dryer. If you have your child read the book, you can have them write their own review, and then you can submit that review on their behalf in your account, with a small note on top saying the review was actually written by your child (or niece, nephew, etc.). Alternatively, you can just use their verbal feedback to help you write your review, but in that case you'll generally want to make sure you have also read the book yourself."

I don't think any of these changes the rules, but it helps clarify that the above things are allowed (and even recommended), since otherwise it might be questionable whether sharing the book with child or having the child write the review could violate other Guidelines/rules such as those about not sharing a review book with others (which in most other circumstances would be illegal piracy).