so i got an email from swimtails (i'm subscribed to their newsletter), and they've released a kids version of their pro line. at the moment they only have one version, the azul, but i'm betting more will be released soon.

i took a peek at the main listing about the tail, and there is this statement at the bottom of the webpage:

Made for Strong & Experienced Child Swimmers

Swimtails takes safety very seriously, each one of our products is tested for the age group that we intend to sell to. When we created our Pro Junior Mermaid Tails, Swimtails in concert with Finis determined that the Pro Junior Mermaid Tail SCUBA Fabric and Finis Mermaid Monofin combination is most suitable for child swimmers. We only recommend that experienced, strong swimmers use our Pro Junior line of Mermaid Tails.

If you have any questions about our Pro Junior Mermaid Tails, please write us at support@swimtails.com or give us a call 1-877-629-7111, we would be happy to help!
what do you guys think about adding kids to the pro line?