RIIM uses IPv6 internally. Child nodes and the Border Router communicate together using IPv6. As for external communications, there are 2 options:
If your internet infrastructure uses IPv6, then child nodes can communicate with an external server using IPv6, no issues.
If your internet infrastructure only supports IPv4, then the IPv6 packets coming from child nodes to the Border Router will be re-arranged into IPv4 packets using a NAT64 and transmitted as IPv4 packets to external servers. The Border Router would register the port number of the node communicating with the external server. This entry in the NAT table is temporary so the external server must reply timely in order for the response to reach the correct child node which initiated the query.