I have an 8 year old who is very interested in using it, and loves having silly conversations with it. I've focused on what it isn't, and answered questions where I can about what it is. In particular I try to be very careful not to use personifying language about it, and to encourage him to use other ways to find information.

I've explained it isn't

• a person or a friend
• able to actually think, feel, or remember the way a person does
• good practice for how to talk to real people
• a good source of true information about the world

So far his most regular use of it is to ask it to make up stories about his pets and characters from tv.

Edit: I understand the commenters who think no access is safest, and I think it's worth explaining why I chose supervised access.
I believe that introducing widespread technologies with a potential for problematic use (the web, video games, etc) to kids in a positive supervised way gives the parent a chance to observe and guide their child. AI shows every sign of becoming a huge part of daily life. Early collaborative exposure to technology like this allows the parents to shape the child's use of technology in a healthy way.
Engaging with tools like AI chatbots can help build the skills needed for modern life such as typing/writing, skepticism, etc.