We tell their parents
Yeah, I coach younger kids so this hasn't become an issue. But I imagine sending group messages to kids with parental consent should be fine. I'll encounter this in a couple of years time and this sounds like good policy to me.
No kids should be on any WhatsApp group - it's an absolute no-no. I coach cricket and rugby and both clubs have made it clear that sending messages (by text, messenger or WhatsApp) to any kids, individually or as part of a group, would almost certainly result in the coach's dismissal.