if its the same reel they use on other kites with the holes just tie the safety line through one of the holes
That sort of negates being able to release or reel in line easily. I think the original posters idea of the safety line slipping over the flying line gives more freedom. And kids want to run with a kite, a stake kills that fun.
Since Dad doesn't mind being there a light caribineer tied to the safety line could be easily hooked over the flying line or unhooked allowing you to "fly" the kite with little impact on the experience.
That's the same idea as my dog lease. Carabiner or shackle with the leash to hold on to.
The kite line will slip thru it and not bind as line on line might do.