I found an article posted to Hattielarlham.org which gives some information on Karla. Here is a short but revealing excerpt:

Hattie Larlham is proud to announce this year’s Family Affair musical guest is Karla from kids’ group Hi-5. Karla will teach Family Affair visitors the new Hi-5 dance moves and song lyrics before taking everyone down under to Australia to meet the new members of Hi-5. This remote video performance will debut the new song and dance exclusively to Family Affair visitors. The cast will also answer questions submitted by fans to the Family Affair Web site at familyaffaironline.com.

If you then go the the Familyaffaironline.com website, you will find the following additional information:

Plus, this is also an opportunity for fans to thank Karla and offer their
best wishes as she embarks on a new adventure. In a “first-ever” remote
video performance, you will meet the cast members, get the opportunity to
ask questions about the cast, television show and future tours, plus many
more surprises! 

So, it appears that the word will officially go out on or near July 29th. I’m very surprised and somewhat disappointed that, with this event only two weeks away, no official announcement has been made.