Look's nice , was it manmade or natural ? Where does water feed into it , just ground water ? Me I would offer some kind of floating pier in the middle away from the sides at least , have it so it's easy for them to get on. Would make them feel safer than basking on land. Turtles feel more comfortable in water or with water surrounding them. This is to entice them to stay. A "DIY" job , some wood that will float , attach a rope to bottom of pier or dock and anchor to bottom of pond with a rock to hold it in place.
Something like but not this fancy :
http://lemonbayconservancy.org/wp-conte ... son-aa.jpg Plan it for two adult turtles , something about three feet. If you need something to help it float how about an old styrofoam ice chest cut the walls and bottom lay them flat under platform see what's needed to help it float then tie together. Ever see at a summer camp those floating piers in a lake or river so kids can rest on and dive off. Something they will be able to climb on. Make it so they will want to stay , little more turtle friendly and it will give you a spot to view them too. Any deer or cattle use the pond , wild predators ... having something in the middle would make them feel safer and more comfortable. If not maybe clear an area so they can bask close to the water , like :
http://l7.alamy.com/zooms/97d70ae8af1c4 ... fa5ty1.jpgJust remember , one day they could leave it to find another pond. Turtles been known to roam a long distance to get to a new home. Any other ponds in the area ? Goggle natural turtle ponds and click on images and see what turtles use for basking site's , the more turtle friendly it is the better chances they will stay.
If it was me I would let them have the pond. Not sure about the cage , if it ever slid all the way into the water they could drown. Also no place to completely dry off and bask properly. Is the pond close to house so you can visit them ? You could offer them a treat when you go. I would not feed them every day and get dependent on you , let them eat naturally on water weed and minnows. You could make a picnic lunch and eat by the pond so you can watch for them and they can see you. If you have a hard winter they may hibernate , this is natural. Once spring comes back they will get more active. One looks a female and can get 12" the other would need a better picture of claws and even if male you could have little one's one day down the road. If so the ponds big enough so they would have there own space . I think they will be happy in it !
Was or is farm land ? Look's like someone gets the Bush Hog out to keep it from going back to the wild. Or is your dad lucky and it's all natural low brush. Reminds me when my dad let me run our 18' bush hog at 12 years old all day. I thought it was fun at 12 , took two days to cut all . We only did it 2 or 3x's a year and I wanted to do it. That is until I got older and found out it was hard work. Haha... Sorry I'm an old retired man , just reminiscing my younger days , they are great memories ! Even the work part enjoy yours , you will be able to look back in 40 years and maybe see your turtle's still there with kids. Yes turtles live some 50 plus years , long time pets !