The plan is available from the manufacturer here: https://www.garmin.com/de-CH/p/779781#overview

The manufacturer does not specify a minimum age. However, it should not be a problem for your 4-year-old child.

If you want to communicate with your child or see their GPS position, you need a subscription.

Yes. But my advice, don’t buy it. It just doesn’t work! Called garmin and they said they don’t sell it here due to network issues