I have heard some illustrators say that they show their work to children and other illustrators that never show their work to children. The picture book market is interesting because adults make the purchase and then children are the consumer. Who knows what will be successful and attract adults and is also “kid friendly”.
For myself I have discovered that a child can give helpful feed back but not in words. When I first show an illustration to a child (usually one of my children) I pay close attention to their immediate response. I watch their face and never explain the illustration. It needs to communicate on its own. I have found this to be good information.
If I start asking them questions, the comments are almost always nice because they know me and want to be kind. This information is less valuable.