That's a very dark turtle. Are you in northern Florida? I'm thinking it might be a gulf coast box, since easterns and florida's have more yellow on their bodies. Of course there can always be unique individuals.
I love the look on Simon's face. Kind of like he was thinking "what is this thing?" or maybe "do you know we have a moving rock in our yard?"
Adult turtles can usually take care of themselves with cats, probably because they'll hiss back. It tends to spook or puzzle a cat.
Is it just a wandering turtle? It seems awfully friendly. Maybe an ex-pet.
Keep an eye on your yard for turtle nests, if you find one you can stick a tomato cage over it or something similar to keep out predators. Native Florida reptiles can use all the help they can get.