Lydia, who has been in the squad for six months, said she got hooked on fishing thanks to her father, David.
She said: “Mum took me to Sutton Bingham where he was fishing and I would sit underneath him, watching the line over my head.”
Mr Thomas said he would be travelling to Idaho with his daughter, who has type 1 diabetes.
“I am going as her health supporter so she can concentrate on fishing and I can worry about her blood sugar levels,” he said.
“We’ve never been abroad and, with the heat, that will affect her blood sugars so it’s all a bit unknown.
“I don’t fish any more, I watch Lydia, that’s where my enjoyment is.
“She’s taken it to the next level.”
The World Youth River Fly Fishing Championships takes place at Snake River, Idaho, from 12 to 19 July.