Create Child Apple ID with Gmail as login ID (or any other non iCloud email)
If you want to create an Apple ID for a child under 13 years old you have to do it on your own Apple device in Family Sharing. One disadvantage in doing it this way is that you are forced to create a new @icloud.com email address and use it as an Apple ID / iCloud ID for your child's account. So if your child already has an existing email address he / she won't be able to use it as an Apple. To make things worse, if you created an Apple ID with @icloud.com email address it is no longer possible to change it (you can do it with no issues if you used a third party email address).